Since the original question isn't restricted to sci-fi subjects, here's my standard list for FineScale's occasional kit polls:

1/700 ships:

USS Langley (CV-1), USS Ranger (CV-4) and USS Wasp (CV-7)

1/48 aircraft:

Fieseler Fi-167, the Germans' purpose-designed torpedo plane

C-46 Commando (I always wished Monogram would have brought this out, when they issued the C-47 when I was a kid)

Curtiss F9C Sparrowhawk

1/32 aircraft:

Curtiss F11C Goshawk

Curtiss SBC

1/20 sci-fi:

I'd like to see Kow-san bring out a 1/20 Nutrocker and Dollhouse, and 1/20 versions of the armored walker kits, like the Greif.

Other sci-fi:

I'd like to see an injection-molded kit of Serenity.  Not likely to happen, though.