Trigger from an HW100. Seriously, it's an incredible trigger and an actual REAL two stage.
Magazine system (complete with anti-double-feed) from an HW100. Indexed by the gun, all metal one piece mags, can't double load and cheap.
Sidelever that compresses the hammer spring on closing rather than opening, from an HW100. Touch the lever and it pops open, give it a tiny rearward click, and close it against the hammer spring going forward. Seems minor, but I find it to feel much more natural and it doesn't seem to move the gun around as much when cycling it.
Crazy to think that when a gun comes with any of these things these days, guys rave about it like it's a breakthrough. Weihrauch did it on their very first PCP from day one, all the way back in 2004. It should be easy enough to do on most guns by now and they would all be much better for it, IMO.