The things we'd see if this game had a modding community like...

[Magis]

Blizz broke the crap out of mods that do that. There was the issue with that one mod that drew "in-world" during ICC in Wrath of the Lich King, but they broke that one, too (it really needed breaking...it just trivialized encounters -_-). That said, the ability to re-skin my UI, control what buffs and debuffs show on my party frames, and resize them...would be amazing.

Indeed, strictly UI stuff is where these addons shine. Another idea I was thinking which Erana reminded me of is a soundtrack mod. Customized every zone to play it's own custom music. I myself would probably replace it with a mix of XIV/XI music.

This of course depends on how much freedom SE gives us coders.

Oh yea, and a tool that actually tries to block gil spammers.