The Tower Defense genre was invented in Starcraft custom maps, all the way back to the popular Starship Troopers maps, then Bunker Defense, and so on. MOBAs were invented in Warcraft 3 custom maps with the original DOTA. It's so clear how beneficial it is to any creative sphere to have those rapid development loops, where you can go from having a cool idea to having random strangers playing it within a few days.

There was another really popular Starcraft custom map (genre, really) called "Commandos" where you had a constant spawn rate of marines and could upgrade them into various things; I always thought that was a super fun mode, but it never materialized into a larger genre like Tower Defense and MOBAs did. I wonder if there's a missed (latent) opportunity there?