I'm still using ST, but there have definitely been times when I wished it would be possible to modify some functionality. [1] I've sent them feature requests, but as they barely manage to implement their own ideas, mine have been ignored. If it were open source, I could've implemented it myself, which I certainly do for other tools that I use and find lacking.

[1] For example the find-replace dialog closes itself when I press "Replace All", as opposed to other editors I've used where it doesn't. It's annoying to have to reopen the dialog after every different search.