This is a variation on the suggestion to allow the player to actually start parties. I've had a few forts where I'd like to actually start a celebration of a major event. Winning a battle, finishing a megaproject, even just 'get everyone together so we can have some weddings' parties for generational forts.
So, along those lines, I'd like to have the ability to call dwarves to a specific meeting area. This way we could gather the dwarves in front of the completed megaproject, into a temple, or around an arena to watch an important fight.
-------------------------------
Going off of the arena thought, I'd be nice if creatures with LIKE_FIGHTING or violent-oriented personality traits could get good thoughts from watching enemies die from a safe place. Soldiers already get good thoughts from participating in battle, why not let the civilians get good thoughts from watching their soldiers/war dogs/traps kill off enemies and dangerous wildlife? This would allow the common player-built arenas to have more of a purpose then simply entertaining the player.
Of course, seeing their own soldiers lose a fight would cause bad thoughts. Maybe some sort of balancing point for battles that are won, but with losses?
EDIT: Changed 'dwarves' over to the less restrictive but more accurate tag-based suggestion. For the top part, assume that whatever sentient race(s) the player has in their fort are subject to the meeting call.