I saw this video awhile back. Steve Fisher saying the goal of modifying a gun is to improve functionality without messing with durability or reliability I think is a good starting point.
While I do think the Glock is probably the best standard bone stock fighting pistol out there it could be a bit better. I like swapping the normal guide rod/ spring out with a spring that has the same tension (or whatever they call it) but a stainless steel body. The primary reason for this is that a little bit of weight up front helps decrease muzzle rise. Some might also argue they are more durable.
Something new I did today is to swap out the standard trigger connector with a #3.5 trigger connector. Between the reference manual and youtube it wasn't too hard to figure out. Took the gun to try at the range today and it was pretty awesome. It's just a bit lighter and smoother but makes a whole lot of difference. The combination of the steel guide rod and 3.5lbs connector is awesome. It is seriously like a whole different better shooting gun. I shot the same gun about a month back and groups were probably less than half the size this time.
Night sights are pretty much required and are an easy upgrade on most modern service type pistols if the gun does not come with them.
I think the combo of ss guide rod/ spring, 3.5lb connector and night sights is an upgrade all of my Glocks will get. Might just stash a spare set or three (at that mythical time when I have $500 for gun stuff and nothing else to buy) just in case. That way I could set up a Glock acquired however down the road the way I like it. Stole that idea from Matthew Bracken's newest book Castigo Cay.
A pistol mounted light like a Streamlight TLR-1 is a solid option. I have a love hate relationship with them. I love the capability but hate the added bulk. For a dedicated home defense or open carry/ tactical gun a light is an easy decision. For a gun that is going to regularly pull concealed carry duty it is a harder call. I like the idea but the Glock 19 with a light kept getting left at home in favor of the J frame. A more concealment oriented slimmer holster would help but that's still carry under a sweatshirt or something, not AIWB which is my preference for concealed carry. I do not think anybody makes an AIWB rig for a gun with a light and if they did I'm not sure it would be tolerable to carry. Definitely better to carry a Glock without a light than to think I carry a Glock with a light but actually pack the snubby 9/10 times. I think running a concealed carry Glock and a home/ tactical one is probably MY way forward.
That's what I have done to Glocks. Things I can see doing
A modern red dot like a Trijicon RMR or the new Leupold offering seems like a really cool way to go. I'm going to wait for some more R and D to happen and prices to slip down a little bit over a few years before seriously considering taking the plunge. Obviously higher suppressor sights would need to go with this setup.
A threaded barrel to go with a suppressor would be cool at some point.
Stippling has potential to help with grip, etc. I would want to see and handle a gun done by a shop before giving them my gun. Too many yahoos with a dremel and a soldering iron think they are gunsmiths these days to trust just anybody offering the service.
Beyond that I cannot think of anything currently available that I want to put on a fighting handgun.
What have you done to Glocks (or I guess other pistols)?