Get a new ASA first, the stock one is terrible and prone to shaving off, and if the shavings get further into your marker it will bugger it up big time.
After that a new barrel wouldn't hurt, neither could a clamping feedneck or QEV.
Over and above that nothing is essential but anything on the ion can be upgraded: trigger, board, breech, body kit, frame, regulator, bolt, the on/off button, you can even put a membrane pad on the board aswell.
Come to think of it, it wouldn't be too hard to build one from the ground up with 'aftermarket' parts, has anyone got a .pdf copy of the manual?