I've come to the impression that for a Space theme to really do well in the future there would need to be some changes to theme, and some context to back it up.
I'll start with context. I think that a manned mission to Mars could be an optimal time to release a new space theme. A manned mission to Mars would be truly historical and heavily impact our culture. Like with the moon, it could really re-new interest in space exploration, especially if we find something wild up there to really hold public interest (Though really what would matter is political interest. I ********* hate how space exploration and NASA are built around political cycles. )
Now onto the necessary changes; I think that any singular space theme would be absolutely fantastic, It might even do fairly well in spite of Star Wars. What we're seeing now though is an increasing importance in context (not historical context like I mentioned in the last paragraph). Star Wars sets do well because Star Wars is popular, but I would argue that LEGO Star Wars also has the benefit of each set having context in a greater universe. If you buy an Empire set you have the benefit of knowing there's someone to oppose your Rebel minifigures. Same deal with the MCU; having that sort of inter-connectedness really contributed to its immense success.
That's not to say that inter-connectedness is the sole deciding factor of a successful theme--but it does spell a good sign for where I think LEGO Space should go in the future. I think LEGO Space should become one singular SPACE theme. This would allow for immense diversity within individual sets and waves. LEGO already has the universe to do it; several years of space themes under its belt has created factions, alien species, and even some characters (Lookin' at you Jimbo!) that could create a really colorful and exciting theme.
It would have a broad appeal but one that's specifically space-oriented. Basically I'm imagining LEGO City, but with Space (not specifically citizen oriented, but rather diverse and varied like City). I can only hope that a LEGO Space theme would have the longevity and lasting appeal of City, but I think it has the diversity and open-endedness to make it potentially so.
The LEGO City comparison brings to mind that we could have civilian Space sets too! Perhaps they'd be less common of sets--if even full sets on their own--but you have to admit there's appeal to seeing a cosmic diner set not unlike the one in the official Space Police stop-motion films! It's exciting to think about getting hover cars and colorful citizens with retro-futurist styles and colors!
Perhaps the most important factor--at least to me--would be that it would allow a serious opportunity for LEGO to remedy the portrayal of Otherness in LEGO Space. It's a toy, I get it, but it's important. It's disturbing to look back at every single Space theme besides Life on Mars and see that in pretty much every instance the aliens were the villains. It would be wicked to see aliens and robots wearing updated classic space uniforms (Space Fleet!), along as being included in other factions. There could still be conflict in every box, but maybe--just maybe just enough kids will notice that aliens and Others can be the good guys too.
Of course the biggest question to ask about this theme would be: Can it co-exist with Star Wars? Truthfully, I have no dang clue. I definitely believe it can--in theory. The key would be to provide something different than Star Wars; offer kids something more colorful and wacky, and way more diverse stylistically.
The theme probably wouldn't even have to be an evergreen theme; it could come back every few years like Pirates if sales aren't consistent. But even so I feel like we've seen Pirates and City (and general themes like them) repeat themselves a lot. This is partly to get similar sets to newer waves of kids, but also because there's only so much you can do all the time. City's gotten really weird with its innovations to the point where it's not really strictly City, but even now we might be seeing some repetition in that (and I'm not just talking about new garbage truck sets).
Sorry for the long post, but I haven't been excited about anything LEGO has made in a while and I'm just spitting my dream scenario. I'm not just a LEGO Space lover but a science fiction lover and it's weird but almost nothing has managed to so fully capture and encapsulate the spirit of the genre (minus darker installments and sci-fi horror) quite like LEGO Space. It perfectly tickles that retro-futurist vibe that gives me life!