Although it doesn't add functionality, I would say that customizing the visuals of the app can truly be a game changer.
Just having the option to change some colors would be really helpful (more nuanced than just light and dark mode. Further than that, using transparency, borders, padding, font change, etc. some of that might go a little to far, but with the right setting options or certain value options in a stylesheet the users could build a theme fitting their visual preferences.
I like rounded cornors and floating stuff, but I know lots of people prefer sharp edges and cornors. Some like to have a background picture, others a certain color scheme.
I would love to customize Thunderbird Android more, because I like the app, but for me I don't like the look.