Theres nothing to upgrade if you're just looking for a reliable handgun. Already has night sights so you're covered there. If its for home defense you may want to add a quality light. I just put a Streamlight TLR-1 HL on my G23 and it is stupid bright.
Anything outside of sights and light for me is just nice to have.
I'd forget the trigger unless you're planning to compete with it. What Gen is it? If not Gen 4 you might want to have the grip stippled. That would be a worthwhile upgrade to slim it up and increase usability.