Two quotes from the Glocktalk forum.
"For pre-Gen5 Glocks, the most common parts to break are the slide lock spring and the trigger spring. Those changed with Gen5 and it remains to be seen what the problem parts will be now. I think the strut inside the trigger spring is one of those parts. RSAs are the most common part to wear out".
"After about 22yrs Glocking, I too thought I needed to stock a bunch of parts for repair.
The only parts I have ever needed to replace were mag springs and a connector in an early production G43.
Honestly I just keep a spare gun for Just-in-case. After adding up the cost of my spare parts, I realized I had spent enough on parts to buy a entire spare gun. I don't have to worry about the newest revision part number or what it fits that way. I have tossed more outdated items in the trash than I have repaired a gun with.
I probably shoot about 1,000 -1,500rds a year though, odds of me wearing out a Glock part are minimal".
RSA= Recoil Spring Assembly. Probably the only part you are likely to ever need. But maybe add a trigger spring and extractor to the list.