This is what I'd love to see from WotC, I have no idea how well it might or might not sell . . .
Continue the prepainted random miniatures line, and use a format similar to the current one. One large or huge mini visible and more random minis hidden inside the packaging. However, it is imperative that each visible mini is so awesome that it makes us all drool with desire. Also, while this visible minis should be somewhat unique and interesting, they should also be something we'd possibly want more than one of. The number of hidden minis needs to be increased. The sets should be small, like they are now, maybe 40 figs, and mooks should be avoided. The minis in the set should be both heroes and monsters, and each mini should strive to be unique and interesting. Heroes should be divided between "generic" heroes (dragonborn fighter) and (in)famous D&D characters.
Secondly, a companion line of unpainted plastic minis should be released, in a format similar to Warhammer from Games Workshop. Turn D&D minis from an accessory line to a full-fledged minis game, and like GW, a lifestyle hobby when you add in collecting, assembly, painting, and playing. The Chainmail game from the 3e days was a good start in that direction. WotC should take on GW! You could buy a box of unpainted and unassembled orc minis for your wargame, and/or for your RPG game. Special characters would be in the prepainted line, but articles on how to strip, repaint, and customize your minis would be easily available.
The two lines would have slightly different focuses, but would be 100% compatible for both RPG and wargaming play. Backwards compatibility for the catalog of existing minis would be important too.
As far as the design of the wargame, I'd love to see a tiered faction system. The first "tier" of factions could be as simple as alignment, good, evil, law, chaos, and unaligned. The second tier of factions could be interesting groups like we had in the Chainmail game. This would provide easy hooks to build your army around, but also provide a flexibility for players to create their own factions and armies.