MOBAs were the top of twitch before. I don't mean to imply its easy by any means to become popular based entirely around twitch - but I do think its very possible to go entirely word of mouth if the games is good and fresh. Publishers have a lot of trouble marketing newer game types - they like to stick to what people know. Demon Souls/Dark Souls is a good example, though From Software does have a long history - just not huge hits in the US.