While responding in the forums the thought occurred to me - not for the first time - that a really great solo-centric MMOG with full offline advancement would be the perfect game to have a multi-character play option.
So, here's the low-down so far to catch everyone up on the concept here so far: a 24/7 advancement solo-centric MMOG where you can have, say, a stable of 5 characters, with open advancment down any talent, skill, job or attribute tree you want; now you can not only manage the open-ended development of a single character; you can manage the open-ended development of an entire group of characters. Create and balance your own team from scratch.
However, you can only form a group if you have the sufficient leadership skills; level 1 leadershiip = one other character in your group with you as leader. Level 2 = some small group buff. Level 3 = another small group buff. Etc. Level 5 = up to 2 other characters in your group. Etc.
When you achieve the capacity to lead a group, you can invite others into your group, and they share in the group buff; however, it also means that you can log in a second character from your stable, and later a third, a fourth, etc. In this way, the developers can still organize some content around the idea of a group taking it on, but the soloer can still tackle that content because he can develop his own "team" by advancing down the appropriate leadership attribute tree and by coordinating the development of his other characters to form a cohesive set of complimentary abilities.
Now, this doesn't mean that a solo player can't eventually get powerful enough to tackle the same content, but this would add a whole new layer of attractiveness and options for the soloer and casual player. The soloer can also take on content as fast or as soon as those who group because he/she will have access to their own group any time they log on.
Perhaps those who have invested heavily in the leadership tree can, eventually, run even more of their characters in their stable at the same time, and focused leaders might even be able to open up more character slots. Perhaps characters can learn group-specific talents and skills.
Imagine logging into the game and having your own well-oiled, carefully balanced team ready to go! No more 60 Ret Pally w/T4 LFG! Or, just taking out a single character for some solo time, or roleplaying a duo, or finding a friend or two and being able to fully flesh out your group from your own stables. Plus, it's a lot easier to fill up a social event and have fun RPing when there are always other characters around to RP to your heart's content - even if you have to do it all yourself! You can put on a play or a show, or develop a comprehensive storyline and have your own characters to play it out through. A handful of players could produce a whole guild of participants.
The possibilities for fun gaming, socializing and RPing would be endless. Why limit the gameplay to just one character at a time?