By buffs I of course mean giving them a tanking and healing tree. Frost seems like a good choice for tanking, particularly after they screwed up and made blood the DK tanking tree. BLOOD IS STUPID! Fact. Fire has a cauterization theme going for it, but I doubt that would get past committee.
While we're suggesting ideas, why not an acupuncture tree for rogues? Maybe a "shadow healing" tree for priests? I'm just saying, priest sounds like a healing class, so maybe it needs more healing options. But if I really had to pick, I'd go with arcane. It just feels right. Frost and ice are hard and slow enemies, fire blows shit up, and arcane manipulates things. For example, Slow could slow down the rate at which you die. Arcane Blast would be an excellent quick heal. Arcane Missiles are really just Penance with a different shininess.
This is obviously a pretty major change to a class that has been around, and been more or less what is currently is, for a long time. And it would create all sorts of problems such as figuring out how to make a cloth tank without needing a whole new brand of itemization. Maybe spirit could give armor, thereby ensuring that the tanks get into fights with the healers for gear, which is something WoW has been lacking ever since paladins stopped using spell power swords.