I was just thinking lastnight, I feel like the Coliseum (aka "the Bloodsands") could really be made into a big, cool feature. In fact, I personally find it kind of odd that the Wolves' Den is the game's central PvP hub, and NOT the Coliseum.
My thinking is that with a bit of an overhaul, the Coliseum could serve both PvE and PvP purposes.
For one, it should probably be made into its own "zone" in Ul'dah. Considering it is one of the most renowned features of Ul'dah, maybe there are multiple entrances from each side of Ul'dah. Obviously, the Gladiator's Guild would probably have an entrance nearby, as well as the Blue Mage NPC's.
Personally, I would love more stuff LIKE the Masked Carnivale, but maybe broken down into either roles, or each fight marking which job you've cleared with. So not all fights might be possible for all jobs, but it could be a point of bragging-rights, like "Oh wow, how did you do that as a WHM? I thought only tanks could do that fight!".
For PvP, I would literally just do an "Arena" style PvP setting. Either small teams (1v1, 2v2, 3v3, 4v4, 8v8), or maybe even have a "Free-for-All" mode? I actually think it would be kind of cool if maybe on Sundays, if you opened up a weekly "Last man standing" type of tournament, with some kind of cool reward to compete for every week!
Something else I think would be cool, if there's some kind of event that allows for "Spectators". Like, maybe for 8v8 fights, you can actually SEE those happen when you're in the zone, so you can kind of enjoy the fights, root for people, etc! I suppose you'd have to disable all abilities while in the zone, though (even if they didn't affect players, it might cause lag or be too flashy and distract combatants?).
Just an idea, mind you! It seems like it would be a fun addition, and might even inspire taking PvP content a little more seriously if done well!