Hooks for plugins
This feature is just an idea, I don't have any particular application driving the requirement, although I think it could be used to allow people a bit more flexibility in implementing some of the feature requests into non-core plugins.
The technical requirements would be pre- and post-xxx hook functions defined around the major functions. Combined with ruby's ability to inject code into classes and redefine functions on the fly, I could create a module that customized the behavior of the system in small ways without patching the code directly.
For example, with a pre-issue-change hook I could implement issue reassignment based on the current state of the issue, so that new issues always went to the same person for triage, then when the bug is accepted it's assigned to the normal user-per-category. Then when resolved it could automatically get reassigned to QA person.
All it would take is some dummy functions that get fed pertinent data, which would allow a plugin writer to redefine for their own purposes.