I think Arkham Horror could have been a great game but is stymied precisely by the cooperation element. We played it with six people, and the cooperation stuff caused the game to slow down to a crawl when everybody had to join in on every decision. Of course, the fact that three of those people insisted that everybody had to read everybody else’s cards so that everybody knew every single fact that might influence any and all decisions… might have contributed.
A guy I know apparently made up a system where you play against each other and against the board, scoring points for closing and sealing gates. That sounds like fun.
I’ve played it and liked it, but yeah, it takes a while and there are quite a few rules. A standing joke is "Let’s round off the evening with a quick A Game of Thrones.