Bill's proposals are very good. However, if there are too many different sprues proposed we may get none.
I have just reviewed my uniform sources for the period and find that in most armies the Dragoons had the same boots as the Horse - the French, Spanish & Piedmontese dragoons being the exception. This undoubtedly reflects tactical doctrines as in many armies the dragoons were used as cheaper, second line cavalry, dismounting only of necessity or in the everyday business of foraging, raiding, etc., whereas the French were more inclined to use their dragoons dismounted in major battles as well.
I also saw that most Horse and Dragoons had a belt over the left shoulder only suspending either a carbine or musket on the right with the sabre suspended from a waist belt, usually under the coat. The use of crossbelts seems to be confined to British Horse and some Spanish, Papal and Piedmontese troops - perhaps some French dragoon regiments also had crossbelts especially in the earlier period.
So, if the rider sprue has 4 figures like the Celt cavalry, the bodies could be split at the waist so that the modeller could use a gaitred leg set or a booted one if both could be provided. This could allow position variations as well if the junction was "cleverly done". As the dragoon gaiters were fairly bulky in this period with full tops, the side buckles could be filed away to make another booted figure with a poor bootmaker ;-) .
The upper body sections could then comprise 2 with a single left shoulder belt, one with crossbelts, and perhaps the remaining without any to provide an officer, standard or musician type figure (but see below re command sprue)
I would also suggest that the figures could be moulded with separate heads, like the Roman & Celt figures. This allows variation in position and head type - most should be tricorne types,and the odd bare head & floppy hat or 2 - grenadier hats if needed could be obtained from "spares" from the infantry set if they were not included in this set.
Any unwanted parts go in the "spares box" or could be swapped with other gamers.
The carbine vs musket dilema can be largely resolved by having separate weapons moulded that can be glued onto the shoulder belt - a clever mould at this scale could even perhaps produce a musket that is easily cut down to a carbine with a couple of strokes of the modelling knife.
Some or all of the hands of the figures could be empty allowing the modeller to choose sword, pistol or carbine/musket to be held - perhaps the right arm could even be moulded separately allowing various positioning (like the Roman set). Cartridge boxes could also be a separate item to allow various positions (Bill, I have little info on just where these were placed - do you have this info, please?) All this means a little more construction, but aren't the cavalry worth it? ;-)
I recommend the concept of a separate command sprue. (Perhaps they could even be sold individually, to allow for different unit sizes, organisations, command figures, etc.)
The officers of this period wore more lace and feathers than rank & file and often wore a breast plate under their coats, and as has been pointed out, also wore sashes. However, a cuirass could be easly added with a little modelling putty over the waistcoat, so isn't essential. There could be a number of officer bodies with some extra arm & head variations added to the sprue so they could be made as officers with sword or pistol- these latter could go on this sprue if there was no space on the "rank & file" sprue. Perhaps one of the bodies could have a cuirass moulded outside the coat. Some of these officer figures could be used as Generals or as mounted Colonels of infantry regiments for those who fancy Old Style big battalion games.)
There may need to be a cornet figure. Almost every squadron carried a flag in this period and it would be nice to have a specific torso with a baldrick (slightly embroidered) to attach the flag to. However, as these were usually junior officers or NCO's, perhaps they could be made from one of the main sprue types with belt over the left shoulder only. However it would be nice to have appropriate Guidon staves. No moulded flag would be needed as there are so many types to be accommodated it would not be useful, & there are so many nice flags out there (Can I plug the Maverick Models flag site again - brilliant)
Musicians are problematical. Certainly a trumpeter on the command sprue would be essential - perhaps a drummer for dragoons as well, though this & kettle drummers could reasonably be bought in metal if the figs are reasonably consistant with one of the major manufacturers who sell individual figures - the problem with Foundry is those very expensive packs! I note the green-up of the Infantry drummer torso - so if Bill's suggestion of some of the figures being in parts were adopted, this could be the way - separate head, legs & torsos - one dragoon drummer & 1 trumpeter torso in the sprue. The latter could be moulded with hanging sleeves that could be easily removed if not needed. That should cater for the vast majority of types.
The other point is that uniformity was not as great as in later armies, so the odd hat or belt being different is not as big an issue. While I know the dragoons rode inferior mounts in most armies, I'm not sure the difference is so great that it warrants another sprue - it would be nice, but...!
I imagine most armies will not field many full Cuirassiers, unless doing entirely Bavarians, Imperials or Danes, so these as well as artillery pieces could be bought in metal - this would deal with the really exotic musicians like Imperial musicians with exotic head gear, etc. Hussars were so rare in most armies that again they can be bought in metal as specials. The main aim is for a generic set and most of the cavalry were Horse (without cuirass) and dragoons.
Looking forward to the Infantry, Tony, and as others have suggested, the Cavalry as soon as possible after as they are as necessary as the infantry, will sell as well or better, and you don't want us wandering elsewhere to get our mounted support.