With no scripting support, adding content like vehicles is pretty much impossible. Blueprints are not sufficient for such a task. A few people implemented them but requires C++ and the game doesn't support downloadable c++ mods. The C++ mods would have needed to be incorporated into the native side of the game but it never came. There is enough content there collectively for a new game but it would need Epic to lead the way. It's a bit too late now for that.

I play UT4 and UT2004 and UT2004 has a bigger community today, 16 years later, than UT4 does.