I don't like how the quest flow closes off completely the second you become the leader of a guild. A couple extra quests for the guild leaders would be nice. I don't think quests have to go on forever, but making it to the top of the faction shouldn't have to be the end of it all.
This is my thought too, although I think there SHOULD be mundane repeatable quests one can perform forever should one choose. (Though not guild related, Povuholo himself makes several of these available -- I'm partial to wine deliveries -- in his excellent Kragenir's Death Quest mod.) There is at least one mod that DOES increase the scope of the Arch Mage position. It is, alas, no longer being updated and not currently supported. I have no personal experience with that mod, since it slightly conflicts with an already-installed mod I'm loath to abandon.
Being Arch-Mage is somewhat contradictory to my avatar's persona, a plate-encased knight-errant who survives by strength of arms and makes minimal use of magic. I can justify his assumption of the Arch-Mage position from a role-play perspective by considering it a purely political appointment. (On the other hand, neither he nor I would ever consider joining either the assassin or thieves guilds.)
-Decrepit-