Steel receiver (usually referred to as a breech on airgun forums...and by Crosman) stabilizes the barrel to various sighting options. (Stace long breech from NZ in the pic below) I consider this a "must have".
Hipac system....allows the use of cartridges or compressed gas at high pressure (hpa, N2) Great little system...really like it.

Trigger grip frame from a P-rod/1701/1720...two stage and SO much better you won't believe it...will need the matching striker as well.
If you keep the stock trigger frame, change out the sear spring for a lighter one...huge improvement.
Trigger shoe...can be made or bought. Spreads the trigger pull force across the pad of the finger resulting in lighter feel.
Do your own valve work....or go with a BOSS valve.
Grips...pick yer maker. Have grips by "Grips by Rick" and "Keiths grips"..both are darned nice.
Rear velocity adjuster to fine tune striker spring preload...nice to have.
The one in the pic has been converted to .25 cal using a Marauder barrel turned to suite and custom bolt. Pic was taken before the two stage trigger frame was added. Keiths "Combat grips" in Zebra wood.

Welcome to the new sickness.
