Being able to create create calendar events from dates in emails benefits from being a single app. Also contacts integration is necessary to auto fill email adresses. It’d be great to have seamless integration between mutliple apps, but with Thunderbird having tabs it’s also a good experience.

I believe Thunderbird supports IRC and matrix, but I don’t use those features. Standalone apps usually provide a better experience.

Hopefully notes is compatible with other notes systems, maybe markdown with a folder structure. This’d compatible with Markor on Android and also many (diy) solutions (e.g. vim).