Beware: this is a big one 
Nota: of course I know that some of these features are available on F-Droid. I’m just speaking here of what could be integrated by default in /e/OS.
1/ [For Fun] MyNews: a personal news hub
Basically, a feed reader + a widget, linked to the Murena Cloud News application.
RSS feeds at first, then - later - the ability to follow YouTube channels, to track forum / social networks activity, to read Newsgroups, etc.
2/ [For Work] VideoCall: a video call and video conferencing solution
What I suggest here is not a complete video conferencing solution, but a frontend (based on a web browser component?) to connect to the major meeting solutions, without any tracker.
With support of Teams, Google Meet, Zoom and kMeet.
A really essential addition for workers. Would be excellent on a tablet.
3/ [For all] Wallet: a FossWallet like application
FossWallet is a very good alternative to Apple Passbook or Android Wallet. An /e/OS native version should, of course, by able to share data with a specific Murena Cloud application.
4/ [Important, IMHO] OnlyOffice mobile… without trackers
A native build of OnlyOffice, without trackers (the official version have some), and connected to the Murena Cloud Drive! It would be a nice addition for workers, especially on the Volla tablet.
Nota: you’ll need probably to add this application to a specific “additional /e/OS apps” section of the App Lounge, as many people do not need a productivity suite on their phones.
4/ [Fennec/Chrome killer] /e/OS Sync: a Firefox/Chrome extension to share data with the /e/OS stock browser
My last proposal is a Firefox/Chrome extension to sync bookmarks and passwords with the e/OS native web browser (via a Murena Cloud account). Then no need to use Fennec/Chrome and a Mozilla/Google account any more.