Literally anyone.
There is an error related problem? Someone can post the issue directly, and if they have programming skills, they can fix it by themselves and then request to make the fix getting merged into the main bitcoin codebase.
So does improvement, anyone can straightly suggest an improvement on this forum or anywhere within bitcoin-related discussion places, for example, as listed on
https://www.lopp.net/bitcoin-information/discussion-forums.html. One of the Bitcointalk members also discussed their improvement proposal
here.
At the end of the day, the fix or improvement request will be thoroughly reviewed by the community.
For an explanation of how the process works, refer to
"Decision Making" Process, and read until "Finding Reviewers".