I recently watched this
Death Battle again. And what do you know? There's a nice little resource to find details about the
Koopa Troop on the Mario Wiki. As is only natural, I figure
40k rules would fit well.
Regular Characters
Since most of the existing bosses are unique characters, best suited as Epic Heroes, I think most regular characters for the Koopa Troop could be elite versions of already elite minions.
Magikoopas, Hammer Bros, Paragoombas, Snifits, Boos, some kind of Bob-omb, big Piranha Plants, and maybe Chain Chomps.
Battleline
Goombas and Koopas 100%. Maybe another based on Detachment, if needed.
Army Rule
I'm thinking something involving Power Ups from the Mario franchise. Mushrooms of all stripes, Fire and Ice Flowers, the Cat Bell...
Detachments
Each would focus on a different branch of the Koopa Troop. They'd be...
Goombas
Koopas
Bros (Hammer, Fire, Boomerang, etc.)
Bob-ombs
Magikoopas
Piranha Plants
Datasheet Examples
Goomba
Koopas
Dry Bones
Snifits