I'm afraid "development" might be a strong word, but yes, I have thought a great deal about this in the past and would love to see some of it get into Final Frontier.
From order of easiest to hardest (there are other things that aren't new leaders or civs we could add too of course):
1. I would love to add a second leader for each existing civilization. The original release of Finaler Frontier had a second leader but I think at the time I elected to disable them until new leaderhead art / flavor text / names got selected. And then this never happened
2. I was hesitant to add more human civilizations, but if you have ideas, then I'd be happy to hear them! Probably not more than 4 though, because...
3. Some ground-work for adding alien civilizations was laid in 1.7 and 1.8 and there's some more code sitting on one of my machines somewhere. The idea was that there would be some races that had already discovered space-flight and were already out there, and others that would be spawned through very rare goody events (the "first contact" event but instead of the civ joining you, they start their own civilization). Think the Mercurians and the Infernals from FFH2, if you're familiar with that, but firing much earlier in the game.
The problem is that I never sat down and wrote out any backstory for aliens and neither did anyone else.
("Race" and "civ" would have ideally been kept distinct, since we already have eight human civs. We could add a race that had been in a civil war with itself for years, for instance. Then we have eight human civs + two civs of $other_race, for instance. Just an example of the kind of thing I was thinking about.)
Adding more civs would mean writing some original fiction, probably, which might also mean writing an official scenario or something.
Are you interested in working on any of this, by any chance? It's unlikely to happen if no one is interested in contributing, even if it's just ideas (normally we have too many ideas and not enough people doing work, but in this case I don't have any for alien races, really). I would say that (new leaders and human civs could happen with very little under-the-hood changes; IIRC the "holographic leader effect" thing is an effect that gets applied to otherwise normal leaderheads so you would just find some leaderheads you like (Either in the vanilla game or from any people have made).
Adding alien civs probably still requires some ground-work to make them happen "correctly". For example, Galactic Civilizations has a "Universal Translator" tech that has to be researched before you can do diplomacy with alien civs. I would happily do more work on the back-end to make this sort of thing possible.