I know Runeterra is the main setting, but with the Void, Darkin, and Targonians, we know that there's beyond one planet. I doubt Riot will explore parallel universes beyond skins, but I'll be surprised if they stop at one or two planets? Are the Targonians trying to conquer the galaxy? Is the Void slowly eating the entire universe? Do the Darkin have a solar system under their control?
I doubt the focus will ever stray from Runeterra
I doubt that another planet will ever become the focus of a story.
And you need to keep in mind that the darkins might have come from a different dimension and not a different planet(and the void is a different dimension).
I honestly cant believe what you just said... There is soooo much trouble in Runeterra that they cant even afford to move to the void! Look at swain rising,Lissandra corrupting, Zyra expanding, The shadow isles taking peolple from valoran ETC ETC ETC! I would love to see the watchers return and I do not care about the void when we dont even know a fraction of Runeterra ....
I doubt we'll go too far into space. There's already so much on the planet that needs to be explore. We also got other dimensional stuff like the Void & Bard.
We already got some space stuff with A. Sol & the Targon Empire (whatever the fuck they are exactly). And I'm sure we'll dabble a little bit more into the cosmos once or twice in the future. But the focus is largely going to stay on Runeterra.
More void plz
There are a couple of avenues to explore celestial and alien champions further in the lore. They could throw in more random celestials like 
and 
who aren't necessarily aligned to a faction (Sol has since been captured but he doesn't count as a Targon champ) and they just have their own roles they play in the universe.
There are factions that could see additions in the future, the 2 remaining Darkin, Targonian Aspects/champions from other planets.
The Targon champs could be Aspects from another planet I think this is a route Riot might explore when the 
rework goes into ideation. Or they could release a champion that is an alien creature that dwells on Mount Targon, based on 
in client lore we could theorize that Targon banishes celestials to Runeterra, in a mist from the mountain. Soraka also being a celestial banished to Runeterra herself.
I don't think we will see any new alien races introduced anytime soon. I think Malphite, Kayle and Morganna are the last old champions that were otherworldly beings. None of which are guaranteed to keep that theme, since Kayle and Morg could also be written to be spirits/ goddesses or Vastayans, and Malphite has the problem of how and why is he on Runeterra, if he is from an alien race.
