@r0se key features i would personally prioritise if building my own:
- syncing files (optionally, encryption at rest- it's all on my home server anyway)
- multiple accounts (i could share the service with others without compromising my own data)
- ability to create public access links (with options for temporary, read-only, password-protected, etc.)
- maybe a markdown editor. maybe.
i do not need a cloud docs editor, i have libreoffice. i do not need an email client, i have thunderbird. etc etc.
caldav and carddav would be nice, but honestly i'd probably build a separate service for that to maintain the "gadget that does one thing really well" mentality