I'm a Monk who only really dabbles in Samurai, but IMO taking out Kaiten isn't the play. It'd be better to just turn Hissatsu: Senei into an upgrade to Shinten, have Gurren be the de facto big damage Kenki skill with falloff, have Shoha just upgrade into Shoha 2 and gain a falloff component, and have Tsubame Gaeshi just slot into the existing Iaijutsu slot based on a gauge or something.

Even if it's just powering up the next skill and the only use case for it is using it on Iaijutsu/Ogi Namakiri, it was a button that felt good. For a game with hotbar combat it had the "feel" of sheathing your sword for the Iaijutsu that you were going to do after it, on top of it acting as a check on preventing you from just spamming Shinten.