Minis for 2300AD:Just a couple of random thoughts.
For aliens for Traveller and 2300AD a miniature company that features "Furrys" from anime' may be able to be adapted to some aliens.
2300AD starships would be easy to make from cheap plastic beads bought from a store that features crafts (Micheals, Hobby Lobby, ect.), and then pinned, epoxy puttied and glued together. A coat of primer and a paint job will be all that is needed to pull it together.
Dittos on the Ground Zero Games. Some of those vehicles and troops are direct ripoffs of 2300AD designs (Rommel/Luftkissenpanzer IX for example.) They are also cheap for packs of figs and have many different, multi-ethnic groups. They are true 25 mm scale and won't mix well with other figures. It's a pity Geohex stopped carrying them in the states.
On a side note, I have gathered some of GeoHexes' terrain and have purchased some resin scenery (Armorcast)to make an alien world like Aurore. I'm painting the GZG figs I have, to camo them in the blue/gray vegetation scheme of the planet/moon.
Kafers...I know how to sculpt figs and cast them so I can make a body and head unit and several different positioned arms and legs plus guns and equipment. Cast them in pewter, pin and glue and paint. Instant bug salad.
I never have liked Kafers though. It seems like GDW could never get away from its origins as a wargaming company.
The other aliens in the game are interesting but I don't believe they were ever explored as much as the Kafers. It's a pity but that's what happened.
End of Rambling,
Lord Iron Wolf