As far as custom ui goes, I would mostly want timers for buffs, debuffs, and cooldowns (yes they are already displayed but I dont like looking up at the top of the screen to see how much longer Heavy Thrust will be up, for example), boss abilities (deadly boss mods for wow, look it up, its awesome, and nearly every (if not every) raider in wow uses it) and a dps meter (I realize that will give ammunition for arseholes to trash players, but arseholes will be arseholes regardless of what ammunition they have, and having a solid tool to see how much you have improved via gear/leveling/rotation is a very neat thing to have, and it lets you see if someone is seriously underperforming in a raid (I don't mean doing less damage than the rest of the dps, I mean doing half as much or less, a player like that is going to be a huge detriment to any kind of end game raiding)

The other things Limecat mentioned would be nice as well, though I probably would not go much beyond cooldown displays and dps meter