I agree completely with the Traveller and generic SF stuff in 28mm.
However, on the day I wake up with the ability to sculpt, I'll do 54mm post apoc and SF. These wouldn't neccessarily be for gaming, more for something to put in the display cabinet.
There are hundreds of historical 54mm figures out there, quite a lot of fantasy, but precious few post apoc and SF - feel free to correct me, please(*)
As to figures, there would be a certain blonde vampire hunter, zombie hunters and various science fiction types. I'd also make the poses fairly static/heroic, as for some reason, sculptors seem to want to do very dynamic poses for SF, which puts me off. GW did this with their inquisitor range - I would have bought a heroically posed space marine, but the pose they used put me off completely.
Early night for me tonight, tomorrow may be the day that it happens

* I have a few from Andrea and Pizzaro. KnightModels do some post apoc, but they're 75mm and the Star Wars range is, well, Star Wars
