On top of what’s been already mentioned I’d mention Far Cry 5 and its arcade, which if I understood correctly back at the announcement, includes assets from FC3 and FC4 to use.
If you are into MOBA you can use Source 2 for Dota.
The latest Gears of War has a very easy tile-based editor you can play with (for that coop escape mode only?).
Trackmania used to have an editor, so check if is still the case if you like racing games.
If you want to learn UE4 I think Insurgency has an editor, I’m pretty sure they had a mapping competition.
Theres also a WW2 multiplayer game (UE4) that comes with an editor, can’t remember its name unfortunately.
