When you're level 30 and have no one to group with, you could get bored. Thieves, dk's etc in a less degree, bashers and healers probably more. Allowing multiing as a possible solution for it is obviously not a great idea (for me). A small idea is
1) another category of rather powerful charmies which occupy slots in group and are displayed in 'gro' command output. They could be gained as preps, or be bound to worn items somehow, etc. So you won't be overpowered when you have a 10 man group, since you can't use these charmies, but will have more stuff to do alone. There could be even more restrictions, like charmies refusing to deal with more than two stinky goblins, and so on. And ofc level-restricting of usage. Or it sounds too complicated already?
2) more single player zones.. Well, general idea is: there're people who are addicted, and they often go bored. Give them a pen and make them to create such zones :) And delete flag retired every hero who is lazy.
Zones where you are unable to enter/port with more than 1-N characters, and probably intended for different classes (like warriors). (yes yes i know you can solo 75% of mud with your warrior).