I managed to get soft-locked in one portion of Deep Sea Zone act 2. I got into a part of the level that usually only Amy or Knuckles could get to, but I couldn't leave because I died once and the door behind me closed. I couldn't use the spinny thing to open up the door Knuckles probably could get to, and I couldn't get enough height with the yellow springs like Amy could.
Also, kinda wanted to be able to summon a big tornado projectile, like in Kirby Super Star. You can already charge your sword attacks to either go Mach-Tornado or just send a really long-range sword beam, but I feel like charging up and then using the downward thrust could create a tornado. If that feels a bit crazy, you could add a second, really short charge time before slamming the sword on the ground summons the big wind cylinder, but either way it feels odd that the downward thrust doesn't interact with charging your sword at all.
In fact, you could fix the soft-lock by using a charged downward thrust! You could make it so if you charge your sword, then use a downward thrust, THEN hit a spring (interrupting the tornado), you could get launched further, like Amy does! I feel it would be odd if he had the ability to go super far with springs normally, considering the extra jumps he can use afterwards, but doing it this way could be a nice addition.
Aside from that one soft-lock I found, Meta Knight is really fun to play with!