Would you be able to seed life on other planets and if you could, would you be able to play as them in the menu?
What would be the difference between that and just customising a new game to your liking? Sounds like this would be a lot of work for only a little bit of added awesome to the game.
In the space stage it would be nice to have the ability to interfere with other life, like uplift species or terraform planets. If we already have a nice planet scale auto-evo then it should be possible (though there are constraints on how many planets auto-evo could manage at once).
I agree with hh though, if you want to play on a specific planet that would be much easier from the menu.