Thunderbird is already more than a mail program. It supports calendar, todo, contacts, multiple chat protocols etc. Notes are in line with features like todo I'd say
native "Thunderbird Conversations" view for threads. Outlook has this and I guess ever other mail program, even K9Mail has native implementation.
multi-level boolean Filters, example (if title "spam1" or "invitation") and ((mail is older than x days) or (mail comes from this recipient)) then delete
better tablet/Laptop view in K9mail (to make it future-proof, for example a GrapheneOS tablet or laptop-like system
dark mode in mail reader?
fixing sensitive information sent through the autoconfiguration
Microsoft's email service isn't really following standards like IMAP/POP anymore and this breaks many apps. So they need to add support for their custom alternative system.