default.nix

(import <nixpkgs/lib>).evalModules {
  modules = [
    ./generic.nix
    ./specific1.nix
    ./specific2.nix
  ];
}

generic.nix

{ lib, ... }:
{
  options.releases = lib.mkOption {
    type = lib.types.attrsOf (
      lib.types.submodule {
        options.variant = lib.mkOption {
          type = lib.types.enum [ ];
        };

        options.someGenericOption = lib.mkOption {
          type = lib.types.int;
          default = 0;
        };
      }
    );
  };
}

specific1.nix

{ lib, ... }:
{
  options.releases = lib.mkOption {
    type = lib.types.attrsOf (
      lib.types.submodule {
        options.variant = lib.mkOption {
          type = lib.types.enum [ "specific1" ];
        };

        options.specific1.someSpecificOption = lib.mkOption {
          type = lib.types.int;
          default = 0;
        };
      }
    );
  };
}

specific2.nix

{ lib, ... }:
{
  options.releases = lib.mkOption {
    type = lib.types.attrsOf (
      lib.types.submodule {
        options.variant = lib.mkOption {
          type = lib.types.enum [ "specific2" ];
        };

        options.specific2.someSpecificOption = lib.mkOption {
          type = lib.types.int;
          default = 0;
        };
      }
    );
  };
}