Marill didn't punch people. He ran into them.
But I could see Knuckles being a very powerful and deadly AT. He wouldn't be fast and wouldn't rack up that much damage, but he would kill relatively easy with some powerful punches (kinda like Knuckle Joe... Knuckles, Knuckle Joe, connection?) Maybe he could glide around for a bit, just kinda hitting people like a fan, and then he lands and unleashes a beat-down.
Also, it would be cool for Tails to be a "recovery AT". He follows you around, and if you get knocked off he will pick you up and fly you above the stage, saving you a jump and attack. Thus giving you an advantage against your opponent who was so sure he could edgeguard you.