I would love to see an infinitely scrollable calendar view in Thunderbird. Two popular applications that already implement this is Apple's Calendar (on both iOS and macOS) and Fastmail's Month View.
I especially like Apple's infinitely side-scrolling week view, and as ridiculous as it sounds, this is the primary reason why I haven't switched to Thunderbird yet for my daily use (I am already a passionate, daily Firefox user). It has a lot to do with my specific needs and how I interact with the calendar app, but I imagine this could be helpful for many others.