The main things IMO are an upgraded sun shell like the Beast, and converting the reaction carrier to roller bearing versus thrust washer.
Depends on the year as to whether it has some of the better parts already or not.
2-4 servo upgrade, an input drum that isn't haggard, kevlar 2-4 band, 5-pinion plantes etc. etc. It just dependson how much power and how deep your packets are.
I used to know those things really well, but it has been years since I built one. There are some new parts out there that seem to address the 3-4 issues, but they are quite expensive. Once could probably do a 4L80E swap for the total cost of the really nice upgrades for a 4L60.