Personally, I would enjoy a MMO which practically requires mutualism, but is also a persistent world where structures are player-made. If you lived in a community, not only would you be hanging out with nearby people, which means that you aren't hooking up with random strangers, but you can also help them out. Moreso, it would be good if low-level people can also contribute to other people. There are things that high level people can do that low level people can't, but there are things that both high and low level people need to do.
Yeah i think this was kind of my point a little. I believe that a successful MMO needs a strong community. I believe that a strong community requires that people help each other and that not everyone can do everything themselves. I don't like the multiple talent trees so that healers and tanks can be DPS to level etc. Now a healer doesn't need a group a tank doesn't even have to tank anything until the max level. Making classes have special utilities that other classes don't have is vital to creating a community as well because it forces people to work together.
You make an interesting point that maybe lower level characters should also be able to help with something. I'm not quite sure how that would work but I think it fits, I mean in the real world you might not a 5 yr old helping someone with something but once people get to 20-30 they can start helping people who are much older. Not sure how the mechanics of it would work but the idea is pretty solid.