So it'll be a self-hosted webmail service with a custom web server that runs locally?
That's too bad, in a way. One thing I'd really like that Thunderbird doesn't offer today is to separate the mail receive/store/send functionality from the reading/writing UI, so I could run the former on my own server and access it from any device on my network.
Obviously there are some relevant standards here and you could set something like this up with enough technical knowledge using other tools that already exist. However, that reduces the potential market by a few orders of magnitude to those who are sufficiently expert with those tools, and then probably by a few orders more for those who have the time to actually do it.