I think KI4 should be an amalgamation of the best mechanics from KI1 and KI2013 (to an extent KI2 but I’m biased towards KI1 SNES/2013 mechanics with KI1 arcade aesthetics).
KI2013 really suped up the identities between universal mechanics (auto-doubles, linkers, manuals, etc.) and unique character exclusive properties (airdashing, command grabs, etc.) because everyone in 1 was a hybrid charge/motion input character with projectiles except for TJ and Jago. The only qualm that I had with 2013 years back (and still do) is that heavy linkers are executed by holding light or medium buttons after inputting the motion. In KI4 I hope they change that to pressing HP or HK and instead complete an ender by motion+holding HP/HK instead. That would solve two things. It would make execution of the universal mechanics easier for beginners because they’d have to only hold a button once ever in a combo and the game itself would follow it’s own rule more than tackle its exception to it.
As far as roster amounts go, I think a large established cast with many newcomers is a good way to go about future KI games. Think of how lacking the KI side of a crossover game would be without a legacy of KI characters!
One more thing. CREATE-AN-ULTRA mode. Like the create-a-trick editor in Tony Hawk’s Underground but you have access to all of a character’s normals and specials. There should be a hit and time cap to make sure troublemakers don’t prolong a match indefinitely but other than that, the player can go nuts.