Add renovate bot (self-hosted or not) or a similar not to your nixos repository to automatically update your lock file. Enable automatic system rebuilds (not live, nixos-rebuild boot...) to keep in sync with the repo.

Include multiple nixos systems in one repo, then reuse configuration or even make them reference each other (if you want that)

Find an issue to report upstream (or even add a pull request)

Fully automate your reinstall using disko and nixos-anywhere (don't forget luks)

Be happy (optional)

Go over everything you've written in your repo so far, realise it's formatted wrong and spend 2 hours fixing it until it no longer works

Build a derivation for something that doesn't exist (also add a pull request if applicable and you've got time to maintain it)

Add a little nixpkgs-unstable, as a treat (use overlays)

Backups

~~Build a server so convoluted, kubernetes is easier to manage (I am here)~~