It would be fun to have a basic enemy pack. Have all the stage enemies to augment your armies.
Id love to see the Octopus Battery and others enter the fray.
Oh! Like Picket Man, and Flyboy. That'll be cool! Other boss monsters like the Guts Tank, Mechazaurus, and Metall Daddy could be included in later potential pack iterations.
What would also be cool would be to have a dedicated miniboss pack including the likes of Friender, Gorilla Tank, Tama, or my favourite Suzak & Fenix.