If I was a Dev for one day, that wouldn't be enough time But if it was possible:
I would
1) fix the minor bugs in game.
2) also change more textures throughout the game
3) also update the game-engine to more modern standards (perhaps even a complete overhaul using Unreal, CryEngine or Unity)
4) prevent players who are in mini-games, siege-war etc. to be outside after a dc
5) rework the so-called dead class combo's like priest/warrior, physical dd d/w etc. so that these combo's can have a use inside ini's.
5) rework some ini's so that the so-called dead class-combo's can have their place inside a party
6) bring in more skins for armor, weapons and mounts.
7) start a new (weekly/monthly) event "GM for a day", so that players will know (and find out) how much work there is for GM's!
8) make crafting weapons/armor/acc worthwhile, by implementing special skills to enhance weapons/armor/acc
9) every thing else I can think off, but not at this moment.
If only it was possible to implement this in just a single day