I am sure there are tons of mods for Glock's. One of the things I like about my Glock is it is so dang easy to take down completely. With that said, I bought mine in April. And immediately started upgrading.
First the DPM Recoil Kit. Better two stage guide rod and the spring has to be twice as long as stock. DEFINETLY improved the recoil.
Next was the Glock Ultimate Pyramid trigger kit from the Glock Store. Wow. Titanium rounded safety plunger, skeletonized striker and full trigger assembly. The only travel now on the trigger is the take-up on the safety. I caught a deal on Memorial day weekend - Normally $249, sale was $189.
Anyway, the only thing I would like to do next is get the tritium tru-dot night sights like I have on my CZ (they help my poor old eyes..lol)
Here is the recoil kit and the trigger kit.