I'm currently starting up a Roll20 game of Spelljammer in 5E. Actually, we've played one session, but we're running through The Sunless Citadel first, so I have time to work on Spelljammer specific mechanics, but here's what I'm doing. Note: I'm running a self-contained solar system. Several planets, but no traveling to Forgotten Realms, or Krynn, or anything like that. It's not traditional Spelljammer, but based on the Shadow of the Spider Moon mini-setting from Polyhedron magazine back in 3E era. We're also playing in a vaguely Firefly-like manner, in that the crew is trying to keep the ship afloat monetarily, so they have upkeep costs and need to take jobs to keep things going, which will eventually lead into bigger plot stuff.
Navigation Tools become super important. Proficiency is going to mean the difference between a trip between planets taking a few days or a few weeks. Travel time impacts supplies and timeframes for jobs. This keys off of Intelligence. Flying a ship between planets goes off Int (navigation tools).
Vehicles (spelljammer) is an available proficiency. If you take the Sailor or Soldier backgrounds, you can take that proficiency instead of the other vehicle proficiencies. I also give out one free proficiency for tools, vehicles, a language, or similar, so they can get it that way as well. Flying a ship is a Dexterity check, but only applies on a small scale, like ship-to-ship combat or docking under bad conditions.
I'm also working on Spelljammer specific backgrounds. Ship combat is going to be fairly basic to start: ballistae or magic between ships. I'm using Eberron's rule of casting cantrips with a two-handed implement doubles the cantrip's range, so ships with casters are going to generally have a staff or two sitting around onboard just in case. Specific rules for 'jammer hit points and stuff, I haven't gotten to. I assume most of the time, the enemy ship's crew is more dangerous than the ship itself. And who doesn't want to capture an enemy spelljammer from time to time?
Honestly, those are the only big rules elements I'm seeing right off the bat. Maybe some maneuverability stuff for spelljammers once we actually get into ship combat, but that's about it. Should be fairly simple.
