Though in WC3 (and SC1/SC2) IMHO this was enabled by a very easy to use «map» editor, combined with a tremendous popularity of the base game.
So if even, in theory, you can do even more (libre software), in practice what you see is not as ebulliently diverse.
You can also compare it to the Spring lobby games - Zero-K has improved on the presentation, but something has been lost by not being able to easily fork the game yourself, build on it, and then just put up whatever the resulting mess is, start a game, and it being listed in the list of games :
Which shows another tension in game development : more recent Spring-Recoil games disable that feature outright, because of the balance headaches it causes, and potential modders can't use what they are not aware exists !
What? Custom game modes is ABSOLUTELY on their radar. Less so than the main game for sure, but to say ZK doesn't support custom game modes is flat out wrong.