While I love Thunderbird on my desktop, I would love to see it work better on smaller screens (or resize better on the desktop too). In addition, there should also be a focus on the possibility of touch operation, even with an on-screen keyboard.
Why? Linux is no longer just the classic desktop. I now use a Linux tablet, for example, and would like to use Thunderbird on it. Linux smartphones are also starting to develop. Windows tablets are also becoming widespread, not least because of the MS Surface devices.