Gentlemen, I'm afraid this will be rather long, but hope it generates plenty of discussion & will bring in the members of other L&UL 18th century submissions, and finally produce lots of figures.
I feel that the SYW (& WAS & French-Indian War & even War of Polish Succession & Colonial wars in India) is a viable option for WF to consider. It certainly caters to those who like big units and has heavy, medium & light cavalry, wider use of artillery as well as light infantry to supplement the line & grenadiers. It is a period eminently suitable to Table-top teasers a la Charles Grant, etc.
Tony, you would need to do a number of sprues, but the potential is enormous, especially as the mid-18th Century is the chief period for those who like Imagi-nations and also Big Battalion style games where 48 - 60 fig units are not unknown - plastic would be ideal for these players. The variations in uniforms would be of little consequence to such players as they often make up their own uniforms anyway. I know of a number of players who would do this type of gaming if cheaper figures were available ie plastic. Also, just look at the effect that cheaper but good hard plastic 28mm Napoleonics have had on that period - we are seeing a revival of 28mm Napoleonics and the advent of "Black Powder" & "Republic to Empire" rules to name but 2 sets that favour bigger units - and one can go back to Peter Gilder's "Grand Manner" rules, Grant, Featherstone, etc.
Now, to the "nitty-gritty".... Reviewing my uniform sources (Mollo, Pengel & Hurt & Ospreys) I could see the following sprues, that if done subtly, would allow conversion to a huge array of units & still satisfy all but the most pedantic modeller (who will likely use metal anyway). Details not needed during conversion can be readily removed by filing or carving, small extra detail added with modelling putty or sprue. The proviso remains that the range should be size compatible with your WSS range & at least one of the major "metal" companies for the period to supplement with speciality figures.
Infantry sprues - I think this can be simplified to 2 sets - what I will call the Prussian and Austrian sets. I would limit poses to the advance musket at the shoulder pose (like WSS sprue) to simplify things, and have the half figure as in the WSS set for command figures, etc. Generally I feel lace can be painted on (perhaps using Bill's talc powder technique) rather than moulded on. Buttons should be regularly spaced and if really necessary can be cut or added or painted on as desired. Coat & waistcoat pockets should be horizontal as a default - the pedants can modify these if desired, but this is the commonest pattern. All infantry of the period seem to have worn above knee gaiters. Accoutrements also appear pretty universal - there is a belt over left shoulder supporting a cartridge pouch on the right hip with a small epaulette on the left shoulder.Hanger sword and bayonet are suspended from the waist belt (a sprue extra could be small waist pouches for those units that had an extra belly pouch ammo supply). There could be a haversack & water bottle hanging over the right shoulder to the left hip, though this detail is probably not necessary. Drummers generally had a wide belt over the right shoulder and swallows nest type shoulder epaulettes. Officers had waist sashes & gorget plates & spontoons, while sergeants carried small halberds &/or canes. I will mention heads below, but perhaps sprues of alternative heads would be another option especially for the less common grenadier types. However I feel that most should be able to be accommodated on the basic sprues as separate swords would only be needed for the split figure
The "Austrian set" I would see as having a coat more generously cut, with medium-sized cuffs, coat turn-backs, lapels to the waist, with rounded upper ends over the collar bones allowing modification as desired. Perhaps a lie down collar as well, though not really necessary as this was uncommon in most units & could be painted on the few. The waist-coat would be somewhat longer to about upper leg-lower crutch level. Head variants should show a full tricorne of Austrian style, a fur fronted grenadier busby with hanging bag (perhaps 2 types 1 with a grenade & no front plate & 1 with a front plate ), a Russian mitre, and a British grenadier cloth mitre. This set would allow most French (WSS type can be used for coat without turnbacks), Austrian, British, Hanoverians, Russians & Franco-Austrian aligned Imperial Kreis contingents to be modelled with sufficient authenticity for most gamers.
Hungarians could be converted using sprue or modifying leg detail, though I suspect that most might buy a metal unit for these chaps. However, if Tony is really generous, a Hungarian type body sprue could be made allowing Austrian and Russian Pandours, Freikorps v. Kleist, etc as well as the Hungarian Infantry - this sprue could have a variety of heads and would be useful for light unit conversions.
The "Prussian set" should have a more "closely tailored" coat with square-ended lapels, lie-down collar, smaller cuffs in the Brandenburg style. The waistcoat is shorter coming to upper pubic level only. Accoutrements as above & long-gaitered legs. The head selection should have the Prussian tricorne (with the front arm closer to the head - like the Brits of AWI or French Revolution tricornes), Prussian grenadier metal fronted embossed mitre, & the shorter Prussian fusilier mitre. Perhaps this is where the Russian Grenadier mitre might go if it does not fit on the "Austrian" sprue, but this would not be ideal as the Russian uniform favours the Austrian cut, at least in the earlier period. This set will model Prussians, Saxons, Hessians, Wurtembergers, Brunswickers & remaining Imperial Kreis regiments.
The cavalry really needs 3 different sprues, as well as the generic 18th century horse sprue that I hope is coming for the WSS. Separate saddle cloths as per Bill's suggestion for WSS would allow shaping of the holster cloth or the rear of the saddle cloth to suit the variants.
The first is for Cuirassiers. This could be used for all nations as the tricorne shape appears to not be such an issue. Elite company heads with grenadier caps could be borrowed from spares in the infantry sets. The horse furniture seems pretty standard as well. The main difference was the sabretache worn by Prussian units and this could be moulded as a separate glue on option. As far as I can determine the vast majority of cuirassiers only wore the front plate in action, even if they had back plates provided. Otherwise the armament would be a musketoon/long carbine slung to the right on a wider crossbelt and a cartridge pouch on a narrower cross belt to left - all with straight bladed sword generally slung from the waist belt. The cuffs are medium sized and gauntlets are shown in many cases though seldom covering the top of the cuff & any buttons there. These could be filed back at discretion. Coat tails have turnbacks. Top-boots to just above knee, not as large as WSS cavalry boots.
The second cavalry set is for the medium types - Horse, dragoons, dragoon guards, even light dragoons. The coats are a fullish cut, with medium sized cuffs (like the Austrian Infantry cuff), turned back lapels, There is a medium length waistcoat with a waist belt that suspends the straight-bladed sword under the coat. A wide cross-belt over left shoulder suspends a cartridge box & swivel for the carbine/musketoon ( mould separate item to allow different placements - perhaps some alternatives could have a tethering stake attached). The right shoulder should have an aigulette and the left an epaulette holding the crossbelt - again, these can be removed at discretion. Cavalry boots were to just above the knee same as the cuirassiers. Heads options should include tricorne, plain bearskin with bag and tassel and a French dragoon/English light dragoon helmet with turban & horsehair crest. Perhaps an English grenadier/fusileer mitre as well to allow the Scots Greys & Hanoverian horse grenadiers to be modelled. Musicians are drummers with swallows nest winged epaulettes.
Again the French dragoons provide a quandary - they are the only types to wear gaiters still, and have their stocking bonnet. Still, a suitable head & judicious filing should do the job. However, they are almost identical to the WSS types, even to the turnbacks on the front of the coat, so if we could have the one sprue for these (pretty please, they could do both eras ;-) ), and the turnback could be filed off for the earlier period if desired.
The third cavalry set is Hussars. The uniform for these chaps was pretty much the same in all armies with the main difference being headgear. They had braided dolmans, barrel-sashes, breeches with short hussar boots & schwaraden (can be suggested by a line on the upper thighs & filed off if not appropriate) and fur-lined, braided pelisses. Weapons were carbine suspended from crossbelt with cartridge pouch on right and curved sabre attached to slings to belt (under the barrel sash) as was the sabretache (again if moulded as a separate item with slings, it can be shaped to suit before gluing). The pelisse could be moulded separately. The head variants need to be busby with bag, mirliton of Prussian type, & mirliton/busby of French type. An earlier Hungarian hussar head variant would allow Austrian grenz hussars & WSS & WAS types to be modelled as well.
Now to artillery!! I note Bill's great work designing an artillery sprue. Could I suggest a variation on his theme. I think that with little quibbling split trailed guns would do for the entire century (indeed so do Front Rank). So could I suggest Gun sprue, limber & horse sprue & different artillery crew sprues for WSS and mid-18th century. These could be sold as individual sprues rather than boxes, so gamers could set themselves up as their rules dictate.
The Gun Sprue: Basically modifying Bill's suggestion - sprue with several trails (2 small & 2 large) & Gun barrels (2 small 1 medium 1 large 1 howitzer) per sprue. I feel a smaller trail & barrel would do for the 3-4pdr battalion guns, medium sized barrel on smaller trail for 6-9pdr's and a large barrel and larger trail for 12 pdrs (medium barrel on large trail is a nice 6pdr variant for WSS guns). All the barrels could have a modicum of generic embossing. One Howitzer barrel would do - Russian specials will need to look at metal I think, as would the bigger 18-24pdrs used in some set piece large battles. Any spare space could be filled with rammers, linstocks, trail-spikes, ammo chests, etc though with wheels as well this may be difficult and these could be placed on the crew sprue.
Limber & horse sprue - basically 2 x 2 wheel limbers with ammo chest and 4 horses. The limber wheels are small compared with those of the guns. The limber could be in parts with an option of gluing a central limber pole or 2 shafts at the side to accommodate single horse galloper guns, fore to aft harnessing or side by side harnessing. A nice touch would be to include a small sheet of thin plastic card to make harness straps with this sprue :-). This sprue could also be used in conjunction with any transport wagon type sprues WF may contemplate in the future covering many eras.
Crew sprues - these chaps are all much the same uniform across armies of their respective eras - open hands in several poses - perhaps split bodies and separate arms - coats to suit era. WSS Crew = longer coat, large cuffs, no lapels or turnbacks & generally no gaiters, though I'm sure an argument could be made that these chaps, of all of troop types, wore these (then they would also do as WAS types and later French as well). WAS-SYW crew = lapelled coat, medium cuffs with lay down collar, turnbacks & gaiters, wide belt over left shoulder holding cartridge/tool pouch on right hip, picker tools in mid chest, hanger sword from waist belt under coat. Extra small pouches for belly pouches and powder horns should be extras to glue on. I would see these crew sprues as being the ones to have the gun tools, rammers etc and perhaps a telescope. Heads would be the fuller tricorne or bare heads - any specialised bombadiers hats could come from the infantry sprues spares. These sprues could also be the ones to fill the role of accessory sprues as suggested by Bill at one stage for sappers, pioneers, pontooneers, etc
Thus I feel the basic infantry & cavalry troops for the mid-18th century can be done with 5 sprues (with 2 optional extras as a "pretty please" which would also have use in WSS as well). Artillery & extras with another 3 (4 counting the crew sprue for WSS) , that will have usage in the whole 18th century, as would the proposed generic cavalry horse sprue from the WSS cavalry suggestions - indeed this could be used in many eras.
I respectfully throw these thoughts before you, Gentlemen, and again apologies for the length necessitated by the detail. I have considered, &wished hard for, this for many years. It has taken the creative talents of WF and the members of this forum to help me crystallise them. And no-one is doing mid 18th century in plastic..yet! A great opportunity for WF to corner this great, and I would think very popular, period for gaming!