As I understand your “requirements”, there is no need for a plugin,
In the sense that a message is in effect treated as a topic (without a category) control over which members can access it already exists.
Earlier you said messages were “hard to search”
Sorry, but I disagree, I do not believe that it would be more difficult for anyone to find any given message than it would be to find any given forum topic.
I guess a large part of the problem is you have said “my requirements” but I do not understand what those are.
Please provide exact and detailed specifications explaining what the requirements are.