Starring Wario, and introducing his tennis-loving brother(?) Waluigi!
A Wario battle of the era wouldn't be complete or grandiose enough without a co-starring role from Waluigi, who in this hypothetical timeline would see his debut in this game, rather than Mario Tennis. Waluigi still hasn't seen a major appearance in any mainline Mario title, which is odd, though at the same time Wario's been fading into obscurity a bit too.
A Wario battle would likely involve him using his newfound abilities from the Wario Land series, alongside Waluigi playing defensively to reflect any projectiles you throw at Wario with his trusty tennis racket.