Pretty much any genre you can imagine was emulated with WC3 custom maps.
Proto-MOBAs and tower defense games were basically singlehandedly popularized by WC3, of course. Some of the WC3 TDs, like Wintermaul Wars, were way more complex than the mobile games you see today, though -- they involved strategies like building truly gigantic mazes to expose enemies to more tower fire and precisely angled walls that took advantage of quirks in the pathing AI to confuse enemies and keep them in the kill zone juuust long enough. Some of them were really unique, too. Battle Ships combined MOBA mechanics with the auto-firing of a tower defense game. Skibi's Castle TD had a ton of unique Mario-Party-style minigames that you'd play against the other players between waves.
There was also a whole genre of reverse tower defense games, where you would buy and upgrade tons of units that would automatically march into the middle to battle enemy units, until one pushed far enough to take out the enemy's bases. Or until there were so many units on the map that the game crashed. But what a spectacle it was.
There were games that were sort of a mashup of RTSes and Betrayl/Secret Hitler, in that every player built villages but one was secretly a "werewolf" whose goal was to convert all the other players.
There was a game called Darwin's Island and other similar ones in an "evolution" genre, which were basically prototypes of Spore.
There were games similar to Civ/Risk played on a map of the world.
RPGs based on popular books and movies were super common, and some were as as good as or better than missions in the WC3 standard RPG campaign. Some the multiplayer ones incorporated a system where you could serialize your stats as a code that you could copy and paste into the next game you played to start were you left off.
There was a CTF-style game called Tree Tag where one player was an infernal who had to tag the other players (treants), who could build defenses and countermeasures as they hid. Then if they got tagged, they'd be sent to jail, where other players could rescue them.
Board games like chess and checkers were common too.
There were platformers like Run Kitty Run.
Even micro-MMOs like Life in the City where you basically just hung out, got a "job", and played some resource gathering minigame. Some of them were crazy complicated, several-megabyte maps with some players playing as institutions and others as citizens. Of course there were fantasy and sci fi themed ones as well.
The best part was that there was zero quality control, because anyone could edit anyone's map, so you constantly encountered variations and remixes of popular maps. Apart from a handful of very popular main maps, you often had no idea what you were going to get.