NextCloud with keypairs would definitely be interesting. The default setup is your classic admin/user hierarchy, but it would be cool to whitelist an npub, and if they can sign their events, they can access the server, maybe even with a little anti-spam zap the first few times.
Discussion
There is mastodon integration, but here we want a sign in to actually use the nextcloud services. This could actually be in interesting route for nostr-native organizations. Maybe a bounty could be set up?