That's a really open-ended question. Depends purely upon your interests and appetite for risk, etc.

Might be worth looking at, from a Docker perspective:

• AdGuard Home (I think it's better than Pi-Hole)
• Wireguard or similar. Great for reaching your services when away from home.
• Audiobookshelf. Audiobooks. There are good apps.
• Calibre-Web. Ebooks.
• RSS feed reader, for non-social media websites you visit. Plenty to choose from: FreshRSS, TT-RSS, Sismics, etc.
• Gitlab CE. If you're a developer or can otherwise make use of version control.
• Gotify. Alerting on your containers. Has a good mobile app.
• Heimdall. A dashboard for everything you're running.
• Komga. If you're into manga. The best iOS app is meh, but the best Android app is awesome.
• Mealie. Recipe database.
• Paperless-ngx. Excellent for storing your PDFs and other digital life.
• PhotoPrism. Basically Google Photos.
• Portainer. Great for managing Docker containers/stacks.
• qBitTorrent. Guess what that's for.
• SWAG with Authelia. SWAG does reverse proxying with a Let's Encrypt certificate, and automatically renews it for you. Authelia provides MFA (Authy, Google Authenticator, etc) on top of it.
• Vikunja. Todoist or Toodledoo without having to pay for features.
• Wallabag. Basically Pocket.
• Watchtower. Automatically updates containers for you. Can exclude the ones you don't want to update, etc.
• Webtrees. Family tree research, if that's your thing.
• YouTransfer. Useful for sharing files without having to use Dropbox, etc.

I have in the past run a Valheim server and a VRising server, too. FWIW.