You could probably combine those last two ideas.

Metal Slug x Fantasy
Metal Slug x Post Apocalypse
Metal Slug In Spaaaaaaaaace
Metal Slug x Mechs

Metal Slug in Fantasy I imagine would be something like that Tiny Tina deal.  A parody of fantasy tropes, maybe with some cobbled together Fantasy-Punk type magic thrower guns.

Metal Slug Post Apocalypse could be basically Mad Max with more of the Fallout Retro-Futuristic stuff in it.

Metal Slug in Space would basically be a cheesy Space Opera.  Mass Effect the Musical kinda.

Metal Slug with Mechs is largely just Metal Slug the Anime, at least in my pea brain.