The roadmap is here https://developer.thunderbird.net/planning/roadmap
17 years ago, a Mozilla engineer called it "the single most braindamaged file format that I have ever seen in my nineteen year career".
Sadly they only mention it in passing (it's blocked by global indexing).
Thunderbird is slowly becoming less dated, but I'm not sure Gmail's UI is necessarily what email client designers should be aspiring to. The former lead designer of Gmail (and Inbox) hated it so much that he has put hundreds of hours into maintaining personal modifications. Now that's he's trying to turn it into a business, the marketing site is a bit more original and there's fewer strongly worded criticisms but I'm sure you can read between the lines [3].
[1] https://addons.thunderbird.net/en-US/thunderbird/addon/gmail... [2] https://addons.thunderbird.net/En-uS/thunderbird/addon/threa... [3] https://simpl.fyi/