Anything from the iconic Ground Pound, to the Star Spin or Cappy himself would make for a great new special move for Mario, imo.
I do think he should at least keep the cape, however, since, as simple of a move as it is and as few of the actual Mario games that the Cape has actually appeared as a power-up in, it's a genuinely useful tool for reflecting projectiles, edge guarding, and, in some cases, even just briefly messing an opponent's flow or attacks up since it turns them around, which itself is what makes it so useful for edge guarding in the first place.
It's not an iconic ability by any means but there aren't many other reflects quite like in Smash, so I do think it'd be a shame balancing-wise if Mario lost it. If any of Mario's special moves should be replaced, it should definitely be FLUDD.