As with any piece of gear there are a multitude of holster options out there and you generally get what you pay for. First of all you need to find a holster that is designed specifically for the pistol configuration you're carrying. In recent years I've seen a handful of "versatile" holsters made to accommodate multiple pistols or multiple pistol configurations. Weapon retention is key and the more specific the holster, generally the higher degree of retention you'll get out of it. Manufacturers like Safariland and Bianchi (same company, actually) are pretty much the gold standard when it comes to duty gear. I'm not saying there aren't others out there but you'll want to vet each option for quality, reliability, availability, and price.
Finally, the big question of customization. I have yet to meet an officer who hasn't had the desire to modify their gun in some way to make it more to their liking. The fundamental problem with this is as soon as you modify any gun it no longer functions as the manufacturer has designed it to. Even things as simple as grip enhancements or additions can alter the way the gun feels and shoots.
Remember those rubber grip covers everyone thought were so great years ago? Most of those wore out over time, turned into a slippery sleeve, and actually created the same problem we were trying to mitigate.
The second most common request is for aftermarket triggers. Although some of these work well and are reliable, most are designed for competition and not duty use. They may feel great and make for a more precise trigger pull, but they're generally less robust than the factory version. If your fancy trigger fails during a shooting match it isn't a big deal. The same problem on the street may cost you your life.
The design and functionality of modern pistols are the best they've ever been and I'd strongly advise you to shy away from making any changes. Remember, good shooters are made through good training; you can't buy your way to a great qualification score.