Support MS Exchange Server Protocols (eas/owa/graph/ews) out of the box in Thunderbird
MS Exchange is a very common solution for e-mails in companies these days.
To access your contacts, calendar, notes IMAP is not enough for a full PIM solution.
Even IMAP is often disabled in favour of more native ms protocols like OWA/EAS/EWS&Graph.
There is a variety of ThirdParty Addons out there to improve the situation, Exquilla, Owl, TBSync, et cetera.
I would like TB to implement Exchange compatibility directly in the core so that maintenance becomes easier and we can convince more users and companies to ditch Outlook.
This would be a unique selling point and improve the usefulness and userbase of TB by quite a margin.