I never liked how you join a game, get rushed when you don't want to, and can't undo the changes in Diablo II (yes I know there are games called "plz rush me!11" but it happens in other "normal" games too :/). I would go further and try to match the players based on their progress. Other than that, you can have an open realm for whatever people want, but the saves would still be serverside for it.