Jojje brings up a valid point. Slack and Microsoft Teams integration would be great too...using extensions (add-ons) for that integration. Redesigning Thunderbird to add support for other applications could end up bloating Thunderbird.
I want to revise my previous comment. Add-ons could handle support for various chat applications. These are listed alphabetically: Google Chat and Google Workspace, Microsoft Teams, Skype and Skype for Business, Slack, and any others .
+100 to that @jojje and @Seamaiden. Perhaps this needs a rethinking of the Chat feature in Thunderbird. Ideally, it should support chat in any form, from Google Chat, Teams, Slack to Google Messages, Whatsapp and Signal. I appreciate some of these might not be possible due to end-to-end encryption or APIs not supporting apps, but it could a great deal more value to TB.
I would gladly see Thunderbird as a strong and stable minimal application for e‑mail, and addons for people’s needs.
This way, dependencies to other services can come and go without impacting the development of the main features and without cluttering the space of people who don’t need those dependencies.
Of course, addons can be developed by the Core team.
Nice one @arkhi couldn't agree more and you're right, begin as a minimal but stable app and grow. By the looks of the UI, it seems that people will be able to configure how they want Thunderbird to work, so this feeds in well.