I'd really like to see Jellyfin support at some point.
Plex is amazing and all, but there are a lot of little issues that are not being addressed and it seems the development team is pushing Plex in a direction that a lot of us are not happy with: i.e. Plex Arcade, their free streaming service, removal of plugins, etc. All of this is being pushed in our faces even after paying for Plex Pass and only takes development time away from what matters most: running our own media server.
Not to mention Plex lockouts when their API goes down (no local auth) and the default streaming quality being at 720p. It's just becoming more and more of a PITA to deal with. Less technical users have a hard enough time being onboarded let alone streaming at original quality. There is also an issue with streaming settings being reset when certain clients update.
Basically, I see Jellyfin overtaking Plex in the next couple of years- especially once client parity is there.
I know Tautulli doesn't support Jellyfin, so I do not know how feasible this is. I know you can view activity streams in the Jellyfin dashboard and there is also a Playback Reporting plugin; but I do not know if this information is accessible via the API or not.
While this may not be possible at this very moment, it's my hope that eventually it will be easy to implement and worth the time investment as Jellyfin matures.