1. PVP - that works, and is a complete and solid addition to the game. Would love to see free for all and team based versions.
2. New weapons and new armor expansions - loved the flourish line, would like to see some more additions in that respect.
3. Expansion on rare weapons and armor that drop a decent percentage of the time off rare mobs - and similarly drop very very rarely out of treasure boxes and related normal mobs.
4. The Core
5. More Artifacts - and have them do something. Whether it impacts your character or not, maybe just give you a glow or a medal on your chest or something to show you have something special ;)
People wouldn't be asking for new bosses if power distortions were rare. The huge demand for new bosses is because the two current ones are so common that they already feel old and redundant.