I gotta say the new BattleStar Galactica on Sci Fi would be my top pick.
I second this but would also like to see 3.75" SG-1 figures. The show's popular enough and in that scale you could make not only a variety of figs but also an "in scale" Stargate.
I also wouldn't be opposed to a 3.75" Resident Evil movie line, or Sky Captain line, or even a good
line from Serenity (Firefly) or Farscape.
Again, I'm a Joe fan(atic) but putting any of these lines in 3.75" scale allows for both vehicles and playsets in addition to the figs themselves.