I don't think any such system can work in isolation. We would need to integrate it with existing email system and SMS and may be also HN/Twitter/G+/etc if we consider reading all these as a to-do item.
One solution that comes to mind is to let users integrate their existing email accounts by allowing them to periodically fetch email through POP and converting it into to-do items. We can also add "Reply as email" and "Forward as email" actions to implement outgoing email functionality.
But at that stage what you want is effectively a messaging client with heavy to-do functionality, rather than some transformative. Bolting the to-do functionality on is probably a lower-hanging fruit than a new protocol.
Ideally you'd need a protocol system that allows for multiple data formats to be specified through a plugin mechanism to cope with things like social networks and SMS on top of email, but then you're squarely in the territory of being at the whims and mercy of constantly ensuring the plugin works with the backend data APIs for the services (for social networks), and sticking to the letter of the RFC law for email.
It's a great idea, but as they say, going to need a bigger boat.
One solution that comes to mind is to let users integrate their existing email accounts by allowing them to periodically fetch email through POP and converting it into to-do items. We can also add "Reply as email" and "Forward as email" actions to implement outgoing email functionality.