Erm, im not apart of any of the three (yet! Heh) but i think since there are three clans on the table and three bosses, i think each week the three should get a different boss, (the illum getting aggy, pillers getting mordis, and pride the troll boss) or just having them assigned to a specific boss
I believe this is exactly what Regenleif was speaking of in his post:
However I think an excellent future rotation could consist of weekly turns of Mordris, Aggragoth, and Hrungnir by Pillars, Illum, and Pride, making every major clan have a boss every week.
Though if I may make an observation, it probably won't be that simple. If you consider two of those clans have yet to kill Hrungnir and no clan in Celtic Heroes has killed Mordris a rotation involving those three bosses is a ways off. By that time, there will be more than just three clans that are able to kill them (I think AnteBellum has already killed Aggragoth) so there could be complications.