I'm knee-deep in the Koopa family tree for a project, and let me tell you, the sheer potential for mechanical designs is just sitting there waiting. I always start from the opposite end of the personality spectrum from existing 'Lings. We've got show-offs like Ludwig, brutes like Roy, tricksters like Lemmy. So, maybe someone really quiet, observant, and tactical. Their battle style wouldn't be flamboyant fireworks or big spiked balls. Maybe something more like battlefield manipulation? A more silent type who creates local gravity wells in the arena that suck in projectiles or mess with Mario's jump arcs. Their airship wouldn't be a flamboyant bass-boosted clown car, but something sleek and stealthy, maybe with cloaking tech that makes parts of the fight disappear temporarily.
Their distinct power could be control over ferrofluid or metallic particulates—call it 'Koopa-Tech' manipulation. They'd be the engineer of the group. Their stage hazards wouldn't be obvious flame jets; they'd be shifting platforms that reconfigure based on magnetic poles they set up, or they could temporarily magnetize Mario's boots to certain surfaces, forcing awkward movement. The personality shines through the mechanics. A quiet, calculating Koopaling wouldn't yell taunts; they'd let their deathtraps do the talking. The visual design could play with sleek goggles, tool belts over their shell, and a color scheme of gunmetal and copper instead of primary colors.
I think the key is making the power an extension of a core character trait that isn't already covered. Wendy's vanity, Iggy's manic science, Larry's... well, Larry-ness. Find the gap and build outwards from a personality seed.