The mods really boil down to what model, gen and the purpose of gun. For example, if your pistol is a Glock 19 that you intend on carrying on a daily basis I would forgo all the extended controls, trigger upgrades and limit my mods to sights, and a beaver tail. I would leave a carry gun mostly stock due to liability and reliability reasons.
Now if you're talking about a Glock 17/34/35 that is going to be at truck gun/plinking gun then the sky is the limit. I would do sights, trigger, beavertail, mag extensions, grip mods, mag well, etc..
The only way I would change the barrel on a glock is if I wanted to do a caliber conversion on an existing pistol or if I needed a threaded barrel to run a suppressor. There is absolutely nothing wrong with the stock glock barrel.
"I wanna go fast" -Ricky Bobby
"Mind bottling isn't it?" -Chazz Michael Michaels