I can’t say I’ve thought a lot about God in Sci-fi. My only suggestion would probably be to just treat it like fantasy. If it’s in this universe, you could put Christianity into it, or you could do something totally new like in a fantasy world that still pointed toward God.
In my mind, putting a sci-fi story in this universe AND dealing with God is rather difficult, or at least gets a bit weird. If you have aliens, you have to think about what God’s plan is for them–did they fall? are they corrupted by sin? does Jesus save them too? I haven’t read C.S. Lewis’ Space Trilogy, but I think it would be helpful for studying Christian sci-fi that takes place in this world.
If your story takes place in a completely different world, then I’d just treat it like Gracie said–like a fantasy world.
Since my sci-fi is in another universe, then I think I’ll treat it like a fantasy. Do you think that there’s a specific type of God-figure I should go for? Or is it kind of a you-decide thing like in fantasy?
I think it’s really up to you. Since sci-fi tends to be more secular than fantasy, I think putting any kind of god-figure in it would be interesting and unique. If you have different alien races, I’d definitely think about the god’s/gods’ plan for them and how they relate to the god(s), just like you would if you were writing fantasy.