I usually do not make any modifications to my firearms except for AR platforms of course. I'd suggest going and shooting it before making any type of changes to it.
Put a good front site on it. or at least paint the front one with some flourescent orange paint to make it easy to pick up. I did the +1 on both of my mags for better control and that extra round.
The first upgrade 99% of Glock Owners make to their gun is new sights. Anything after that is up to you, and how you intend to use the gun in question.
New good metal sights with a squared off edge on the back sight (non-novak style) is an absolute must. If you try to break open a stuck slide with the factory sights or run 1 handed manipulation drills you'll pop the junk plastic factory sight right out of the dovetail. I opt for either Ameriglo Operators or Trijicon HD's. The bright colored front is a huge benefit in picking up your front sight fast. 2nd mod was a Ghost Edge connector. Made a big difference on the trigger pulls of my 43 and 42. 3rd, stippled the front and back of the grip. I didn't do the sides as I appendix carry it, but even just the front and back make the gun MUCH better to shoot, especially with wet, cold & numb, or sweaty hands. Finally, I also undercut the trigger guard. It makes it more comfortable in the hand and helps avoid that all too common callus on the middle finger knuckle.