RSS is great - but it's not email, and doesn't need to be integrated into an email client. Email is inherently two-way, personal, private. RSS and the web are public and one-way. These very different modes of communication are best kept clearly separate.
Personally, I use Usenet, ftp, Discord and other communications apps - but I have not the slightest wish to see them taking up space in my email client, or potentially compromising its security.
If you're using thunderbird to pull your RSS feeds (as I do), then can't Thunderbird itself keep those references as a to-read for you? Why use one application to manage the subscriptions just to forward them on to another application?
I think this could be useful. For example, when you click on a link in email and it opens within Thunderbird to an article, a Pocket button on the toolbar would be handy.
I also think Thunderbird could be adapted to act as a desktop based Pocket reader and store articles offline. As an optional module, for those who use Pocket.