Suddenly, it dawned on me how much I enjoyed such game types: Firefight, Horde, etc. Immediately after, I started thinking about how a similar game type would work in WoW, and I found myself liking the concept. Think about it: anywhere from a single character to groups of up to five players working to take down waves of ever tougher enemies, earning achievements, titles, and gear similar in fashion to challenge mode. Perhaps there could be normal and heroic variants as well.
I'unno, just a thought.