IMO...you can only cater to solo players or coop play. Catering to both will never be equal and will always make one option better (usually solo since it usually stems from convenience) and if one is better than most if not all will walk that road leaving the other pointless.
The POINT of an MMO is coop...there should be next to ZERO catering to solo play. I have a long list of games that are more fun to play than MMO's when it comes to single player. I am in an MMO for one thing and one thing only...human interaction as much as possible. The human dynamic is unpredictable and highly more interesting. Solo games are entirely predictable.
Also I don't suggest heavy power progression either as the only benefit is that players like to feel they are progressing as a character (but there are many ways to do that). It should be light and logarithmic (each level rewards less and less till it's pointless). The only thing heavy power progression creates is a boring/frustrating game, imo. It does many negative things with only one positive:
1) It makes content obsolete which means wasted resources. The less focus on power progression the farther the same amount of content will go as far as interest and diversity of play.
-Once you are in the end game 80% of the game world is pointless and if not just a joke to be in because you one shot everything.
2) Promotes griefers and ganking.
3) Makes the game feel less like a game and more like a job as heavy power differences are substantial so the need to increase power is more important and therefor anyone not at the top becomes a victim.
4) It doesn't give any reward to skill and gameplay is less about fighting and more about increasing power like gear/levels.
For the most part. I believe leveling should die. It's a mechanic invented to simulate character growth in times when there was no other way and I believe programming has come to the point it can more interestingly simulate progression in more meaningful ways than a number and the ability to one shot.