Rogue would be the most optimal choice.
You already have a long range and a melee, so why not add a mid-range? Besides, Rogue is great for solo dungeons, events and rushing.
Mystic would be the fun choice.
Hard class to master, but gives you a great sense of being in control once you do. Even if you’re just average, you’ll already be a great support.
Wizard, as always, would be the most solid choice.
Another long range to farm on in case the Archer kicks the bucket, Wiz also doesn’t need any UTs to be efficient and is not particularly tricky to play.