If I run the spelljammer adventure from the book (Light of Xaryxis), which I would like to do. It's not very long, either. What would be about 30 hours of play, give or take IRL - so, y'know, months and months by PBP, but nothing compared to an AP like Tomb of Annihilation.
The expectation is characters of Level Five, with Background Feats. (Feel free to ask if you don't know what I mean). The idea is (as an introductory to Spelljammer adventure) that the PCs have likely never HEARD of Spelljamming (though I don't see any reason that there couldn't be stories of it). I'd like to make up the starting setting (for now think of it as Generic Fantasy World) for reasons that will become clear later.
So, I'd rather the PCs are from NON-SJ races. If you're really keen on a Giff or a Thri-Kreen, or whatever, we can talk about it.
Generally, all you need to know to come up with a character idea is this: You should be someone who would be near the docks in a fantasy port town on a particular weekday afternoon. A merchant. A smuggler. A bartender or barista. A street vendor or chef. A traveller. A coachman.
You don't even need to be someone who means to be an adventurer (though at level 5, you are very qualified) but, obviously, you need to be someone who would rise to the challenge.