Yeah players are always always going to prioritize themselves over their party members as far as progress goes. This is why I think Portal is so well designed, you are only as far ahead as your partner and they have to do things that you can't. That's what a cooperative experience should be and I think the best solution to making team play important is making team play necessary. It should be too hard to do it solo if it's an online RPG.
As for impact on the world, that really depends on how you define it. Permanent effects are always going to be hard to design around because the parameters can end up more different than you ever imagined, at which point things stop working. You can have long-term effects on the world very very easily though, though I've rarely seen it done.
Like I mentioned before I'll probably be looking at time-gated content dealing with world changes as long-term challenges for the players (with one of a kind, or at least very very rare rewards to reflect them). Those will probably last only a couple of days and happen every week or so.
