You might find the What Have I Missed? and I Just hit 50. Now What? and Where do I unlock 2.3 content? pages on Reddit helpful. You've got several dungeons and Syrcus Tower to unlock, plus some primals, and the second part of Binding Coil if you're into that. You can also do Frontlines (24 vs 24 vs 24 goal-based PvP) for Mythology/Soldiery Tomes and/or Hunts for tomes and Allied Seals (which is also a path to gear).