Its good that there is already project level configuration with dev.nix but I believe Project IDX can use more of the nix features by providing a way to extend the workspace using another .nix configurations that will be only for the user.

Example uses of this is using vim extension, you may use it but it is not required for the project therefore affecting other users or your team that will use Project IDX.
Another is using a specific program that you only use. The list goes on,

User level configurations will be great addition to fully use the customization powers of nix in Project IDX.