*Motor saver air filter
*Platinum Revo shocks
*Bad Horsie shock covers (or some kind of shock covers)
*Titanium pillow balls
*Rear wing
*Rechargeable receiver pack (if yours came with the 4 AA set up)
*Fail-safe and temp gun (if you don't already have them)
*You could get a bigger motor (but I'd wait until the 3.3 was worn out)
*RPM A-arms and bumpers
*Aluminum push-rods and turnbuckles
*After market drive shafts
*FOC (forward only conversion) this eliminates the Opti-drive module and removes 90% of the gears/parts in the tranny. You may want to hold off on this one unless you're comfortable driving it with no reverse.
*Good set of RC tools
These are all just suggestions/ideas, the only thing I'd say is necessary, is the fail-safe and temp gun.