That would allow
* specifying custom Wine builds
* pinning dependency versions
* trivially maintaining and applying patches
* creating wrappers and setup scripts
* atomic upgrades with rollback
* explicit and reproducible dependency resolution
* maintaining your own easily shareable repos with custom packages and overrides
* a platform/frontend-agnostic packaging solution (no need to use any particular GUI - can just manage your games with Nix/Guix directly)
It would also contribute to the declarative package management revolution.
Just look at this list: https://alternativeto.net/software/lutris/ - wouldn't it be great if those could all use the same packaging format and use/extend a common upstream repo?
Guix is ahead of Nix in this: https://gitlab.com/guix-gaming-channels/games
Sunbeam City is a anticapitalist, antifascist solarpunk instance that is run collectively.