Some ideas:
Options and Settings
Option for a contrast-based mouse cursor. I lose track of my cursor a lot.
Option for full-screen monitor selection on multiple-monitor based systems.
Skills and Mechanics
How about giving 1 class the ability to drop some kind of orbs that can be DPSed down the owner for some kind of massive heal?
Here’s an outlandish idea: what if necroes could pull 1 other player into a shadow world for a 1 on 1 duel.
Moving PBAOE-based combo fields!
Economy
Player trading? There’s gotta be a way to make it work.
That’s all I can think of for now. ArenaNet and NCSoft have done an amazing job to date!