I would like to see the Thunderbird Chat implement OMEMO for XMPP messages. Every modern XMPP messenger has OMEMO, and implementing it in Thunderbird would preclude having to have an additional XMPP desktop app just for messaging.
I have a number of non-technical people who use Thunderbird already. It would be far easier to not have to teach them how to use Gajim if they could use an application they're already familiar with for communication. Sadly the lack of OMEMO support currently prevents this.