Halflings with pikes make sense. They certainly work together and have a good sense of cohesion. On the same note, they will probably try to keep the enemy as far away from real close combat as possible, so pikes will form a defensive line backed by slings and archers. They might have combat dogs to do the infighting, and barded cows or goaded boars for break up enemy formations. They will certainly have a good element of light infantry. Cavalry will be few (perhaps only selected characters), riding ponies.
I still have the old halfing firepot from GW, which might be a good hint on the amount of their artillery.
On the defense they will probably use earthworks with pittraps as much as stakes, and rely on their archery to keep the enemies away. Resistance against any serious army will not be substantial, though I am sure they can then raise a hell of a guerilla war.
In terms of miniatures, we would need lightly armed fighters with pikes, slings and bows for staple units - perhaps with the odd shieldholder with some kind of pavise to protect against enemy shot. Heads should be bare or hooded, with the odd pothelm (or just pot) and leather cap. Specialist units would be herders with animals (bulls, boars, dogs, goose), sneaky scouts with knifes and the odd and rare fighter. Don't forget the marketenders, so that we can field some real nasty berzerk halfling (if you come near their rations).
I would really love to see them, though I doubt that these would make to the major selling army of the year.