Let's take a seat, children, and discuss the severe lack of FFXIV music on OCRemix.org
Now, some will argue "90% of the music from this game is just remixed versions of previous games", to which I would say "Well what about the 10% that isn't?"
Personally, I'd like to see some alternate takes on the different Primal Battles. Shiva with pixietricks? Yes please. Titan ala NekoFrog? Take my money!
Any of the specific dungeon themes, as there's generally FOUR songs per dungeon (Background music, Background music during a fight, first and second Bosses, and the final boss), are also perfect for arrangements. Playing around with different dungeon's music and arranging them would be really neat to hear.)
There's plenty of opportunities for really interesting and great arrangements in this game, and it makes me kinda sad that no one has jumped on it in the years the game has been out. And new music is added every three months! It's ever expanding!