What I believe one of the most interesting parts of visiting space would be is Pokémon themselves.
Unless we take any with us, it's very likely we won't be finding any Pokémon there that usually reside in their world; instead, we'll be getting chance of running into brand new ones, possibly having much, different style and characteristics than most Pokémon we have seen before. Due to them being in an isolated area and whatnot, I could imagine their attacks to be completely new as well, and also ability. Of course, some of them could be resembling, possibly, but they would still likely be different in their own way.
Space legendaries could be more fierce-looking if they wish them to, I feel, and perhaps could be even more powerful?