The holy trinity of WoW has been the staple for most games and that would be the roles of Tank, Healer and DpS. Now don't get me wrong I think the current system works great. Clearly it works well otherwise it wouldn't have become so wide spread.
First off lets call my idea the Booner.
By saying I'd like to make a change it would be to add a new role into the current set. Something akin to the Warlord of D&D 4th Ed. who is pretty much a group buffer with tank level amount of dps to add to the composition. By buffing the group I mean they would be able to have short duration spreadable damage increasing abilities, damage reduction cooldowns and healing cooldowns. The role would be there to assist in the ebb and flow of the battle. Say during a raid the raid has a short burst phase the Booner would throw out some spreadable damage cooldowns and the same can be said for burst healing or spike damage moments.
I know they tried the idea of a supporter back during classic and that failed. Personally I say it failed more because people didn't want to be shoe horned into roles they didn't want and the Paladin and Shaman by and large only had the roles of healer. Now by say adding a 4th spec of support for some classes it could be a spec specifically designed to be a supporter class.
I could be wrong of course. So what do you people think.