Some sort of 'escape from the space station?' Or a 'space war?' What version of Galacticraft? If it's 1.11.2 you can p.m me the ip if you want and ill hop over and hook you up with ideas. Just no redstone. Can't do redstone ;-)
I figured out escape from space prison (but I have no idea about the implementation). The second idea is orbit battles with TNT cannon armed stations and oxygen sealers as the only source of oxygen, maybe players could build the stations themselves. And it takes advantage of both zero gravity and the need for oxygen. Plus you get beautiful orbit scenery.
You could use command blocks to help with most anything (Since it's GC4). When more addons are updated to GC4, you could use a lot of them (See the Giant Galaxy Modpack) and host interplanetary warfare! The Overworld will be a kind of hub/neutral planet. Players should have an accelerated start. From the Overworld they can get a Team/Faction's Homeworld.
Each faction would have their own kinds of motherships (maybe not the best, since 2 can't be in the same dimension) that they build to their faction's theme. They can attack other faction's Homeworlds to try to eliminate them. Factions can take over non-Homeworld planets to get some kind of bonus. (Better resources, lighter blocks for motherships, more powerful weapons?). Each planet has 1 base, and is ruled by 1 faction at a time.
(I realize this would be a massive project, but I think it's kinda possible, and cool)
If you don't want to even consider that, I think a Spaceship vs Spaceship battle would be worthy. Oxygen would be needed. You could also have classes and Oxygen recharge pads (from command blocks. Just replace a players O2 tanks with full ones after a moment or two. Tricky part would be putting the Tanks in the right slot)
Space Marine: Has 2 Medium Oxygen Tanks for outside combat. Grapple with lots of string for outside combat. Can't do any construction.
Engineer: Can place O2 bubble distributors, but must hook them up with the ships main O2 pipes. 1 Medium O2 and 1 Small O2 tank.
Again, this is just my imagination running wild, but it could be a great minigame.