Don't reinvent email. Just add tags to indicate TODO fields in the email headers. Then youre building on the preexisting infrastructure. And it can handle the degenerate case where the TODO is to read, since it is email.
I'm no expert, but I'd think some of this is already being done, no? However, there's more value to a few companies to have walled-garden messaging platforms vs 100% open and interoperable, and that's not a problem solved by tags.