It all depends on what your desired result is.
Metal sights are the first additions, several great ones out there...tritium is a bonus, but not necessary.
I detest my glock's trigger pull. I added g17 smooth faced trigger (simply a preference, I like my triggers smooth YMMV), .25 cent trigger job, 3.5 lb connector and 3.5 lb spring. It's better and it works. I'm still not satisfied, but I'm not exactly dissatisfied, either. I could have learned to use it as-is...but it is my gun and if it doesn't interfere with operation, I see no wrong with modifying it.
I also added an extended mag release. It's not necessary, but it is a preference. One of my holsters doesn't cover the mag release. When I wear that holster, the mag release digs into my love handle area. Wearing an under garment helps, but not much. I switched back to factory. If it were shaped like the gen4's mag release, I'd be happier with it.
I added the glock extended slide stop. It is handy for locking the slide. Other than that, not really useful. When I slam in a new mag, I either rack the slide or engage the magazine sharply and the slide drops by itself. I like to have the extended slide stop there just in case I were ever operating it one handed and I needed to close the slide. If it's life or death, then the whole "slide lock vs racking the slide by hand" argument goes out the window. Just a preference. YMMV.
Overall my g19 is what it is. Metal sights, ammo and more ammo are about the only useful mods. If you want to alter the trigger to your liking, use glock stuff since it is reliable and proven IMO.
Edit:: If you reload, replacement barrels that allow you to shoot lead reloads are a useful mod. Especially for those that own 23's and can switch between 9mm, 40 cal and 357 with a few simple part changes. I should have bought a 23 instead.