Hmmm, not too obscure is tough; I LOVE Hydra Miniatures pulp sci-fi line and plastics there would be amazing, but that's definitely a niche for example. Warmachine, as mentioned, really should be investing in hard plastics but that's also it's own niche game, particularly now. I want more of Perry's ranges in plastic but they are already doing plastic. Grey for Now Game's could use plastic samurai to support their aweosme metal range, but they are already collaborating with WA.
So casting my gaze further afield:
Antediluvian Miniatures have an excellent HYW-ish Scots range (bought from Claymore Castings) and plenty of supporting stuff that would be excellent in Plastic.
Crooked Dice Games has a lot of amazing stuff in their 7TV range that would probably work well as plastic kits, though some of the topics are a bit out there.
The Assault Group has a some excellent ranges/eras that would amazing to have in plastic.
More as I think of them.