The better question is, what do you keep breaking? If you arent breaking anything and having fun, then dont do a thing. Othwerwise, lets get and idea of what you want it to do
It just depends on what you WANT to do.....the tmaxx has about the best aftermarket support so you can do whatever you want&can afford to do. If you arent breaking stuff and just want to do a few things to make it better then....aftermarket skids plates are a great idea(the servo skid is not needed IMO.....i have never damaged a servo from any kinda contact with anything) another good upgrade is CVD's and some threaded body aluminium shocks.
But like was mentioned....what have you done so far??