A very popular upgrade is a new feedneck - Such as a Q-Lock clamping feedneck - The stock one isn't great and a clamping feed will hold your hopper/loader much tighter.
As for other upgrades, its really up to you - Best to get it, try it then decide what you think needs changing. A lot of people opt to upgrade the trigger so they can "walk" it faster, but its personal choice and you may feel content with the stock one.
On my Ion I also have a QEV (Quick Exhaust Valve) to increase efficiency, but this isn't vital, and also an on/off for the air supply.
-Will