To start off my mods, I have a simple yet clean mod on one of my all time favorite blasters, the PistolSplat.
It's pretty simple really, as the PistolSplat is a great blaster already and one of the only pistols I actually like. I took out the spare spring in front of the plunger, which reduces plunger velocity. I then replaced the existing barrel with 1/2" CPVC. Pretty standard. I made some other updates too, that I haven't seen done anywhere else. I sawed off the trigger guard because it was uncomfortable and added a 3/4" CPVC outer barrel that nests the 1/2" inside. I'm not sure whether it actually improves performance, but may add a slight advantage in power and accuracy, as it keeps the front barrel straight, guides the dart and channels the air behind the dart. Hypothetically :p