Your idea doesn't sound too difficult to implement but I don't know if people would want to store all these messages locally when the vast majority of people are used to having their shit be stored elsewhere. Additionally, if you wanted to target enterprise users, they would want to likely have all their messages centralised for auditing purposes
Other than that, I think its a pretty neat idea
Man, I really do commend the effort you put into this. I'm so far into cynicism land that I don't think I can find my way back.
I just can't help feel like you're wasting your breath trying to reason with ozma (as is evidenced by his hilarious reply to the same comment I'm replying to)