The idea of guilds competing with each other for cash and prizes is popular. At first it might seem like a good idea to do pvp challenges for all x guild vs. y guild members but I think that would tax the system too much, i.e. cause a lot of lag.
What I propose is that guilds fight each other in a raid format. Each guild would face off against waves of the other guild's toons as pve enemies. The system would create enemies based on the toons in the guild at the time of the challenge. So if a toon is a level 35 Justicar, the system would generate a generic level 35 justi to be part of a wave. I am guessing it would be too difficult for the system to keep track of builds so it would have to do generic builds, some melee and some spirit, etc. Might levels might be applied to the generic toons, if the system can handle that.
I think that facing the other guild's toons in onslaught waves would be a good challenge! The raid could run for one hour and the waves would keep recycling the same toons as need be. To account for different guild sizes, the winner would be the guild with higher ratio of combined guildRPP divided by the number of guidlies participating in the raid.
Possibly, the guild master could be presented as a Boss enemy once all other toons have been defeated and then the waves would start over again. Slay the GM more times for more prizes?
Anyway, that's my latest idea.