Issue number aliases
Redmine uses a global issue number allocation scheme for various reasons.
Other trackers such as Jira support a project specific prefix such as "PROJ-321".
While I'm not asking for issue numbers to be overlapping, supporting per-project issue alias strings will make it easier to identify the project from a reference : commit logs, etc. without having to go to redmine each time.
So issue # 321 could be aliases as "PROJ-321", at least in the URL for external links, and displayed to the user as such on Wiki etc.
For tickets moved across projects, I suggest the old alias just warns the user but continues as the issue number should still be the same.