[name]How[/name] much SF aspect is there to this fantasy? Is [name]Earth[/name]'s history up to now the same? That will suggest how “realistic” to expect the names to be.
[name]How[/name] far in the future are we talking, and how many settled solar systems? I’m guessing a fairly small number of planets will be involved in the story, judging by your need for many places on planets to be distinguished. In a story like [name]Star[/name] Wars, where the world is the galaxy and planets are treated like cities, they have little need for place names on a smaller scale than planets.
I can assume the first settled solar systems will have planetary names taken from mythologies other than [name]Greco[/name]-[name]Roman[/name], those having largely been used up in our solar system. In the 1990s, I wondered what was going to happen; then the discovery of Kuiper Belt objects led to use of names from other mythologies even in our solar system.
At some point, that naming convention is going to break down. After that, I assume planets, and places on them, will tend toward [name]Earth[/name] naming conventions. I don’t expect a planet named We Made It (from a [name]Larry[/name] [name]Niven[/name] story, obviously named for the first words said by the first person to land).
What are [name]Earth[/name] “naming conventions” exactly? Much of [name]Earth[/name]'s names are from the pre-scientific era, and I expect that the methods leading to them won’t necessarily be applied in the future.
I don’t know how most [name]Earth[/name] places got their names.
Place names derived from demonyms (Yugoslavia “land of the South Slavs”) aren’t usable for anywhere with no indigenous population.
There are countries and regions named for geographical location (Australia), people in their history ([name]Bolivia[/name], [name]Colombia[/name]), resources (Argentina “silver country”). The polar regions seem to have a lot of places named for monarchs: Franz Josef Land (islands north of Russia), Queen Maud Land (Antarctica), etc. These are relevant as the last land masses and regions on Earth to be named, but I doubt that style will continue.
I have trouble with this myself, because it looks like there won’t be any really new names, at least until new languages evolve. That’s why the most convincing/stylistically appropriate names I can think of are from consciously “Earth in space” retro universes. For example, a couple RPGs I’ve seen in development. A colonial-era-in-space game had a planet called New Entwistle-on-the-Wold. A Cold-War-in-space game had planets called Potomac, Cincinnati and Wellington, along with the expected mythical names. And Shangri-La, of course. Not a real myth, but… Reminds me of the Honor Harrington universe, a Napoleonic era/Cold War in space series. Its planets are a mix of reused Earth places (Zanzibar, Potsdam), mythical names (often heroes or monsters rather than gods - Beowulf, Manticore - I love this because it implies the god names were used up), historical figures (Yeltsin’s Star), not all political (Rembrandt), literary (Erewhon)
[name]Remember[/name] that interstellar settlers often won’t be European.