New Skill rules?

At its most bare-bones, rudimentary, all-flavor-peeled-away, this is a good idea of what the new and official Shunko skill will look like from a practical perspective (and this is a deliberately "dry" phrasing just to give you all the idea of how it'll work), though it bears keeping in mind that this could change at any time:

Shunko (or the racially-flavored equivalent) allows the use of the Spirit stat instead of Strength for melee attacks. It is activated and maintained much like a release (its duration per skill rank mirroring that of a Shinigami's Shikai), and Expert-level practitioners of Shunko can maintain their Shunko Aura indefinitely, which also removes the combat action requirement to activate it. When the Shunko Aura is active, a character may choose to use either their Spirit or Strength for melee combat, but note that skill checks are still made using Hakuda or Weapon Proficiency.

At Master Shunko or higher, the Shunko skill itself may be used for checks instead of Hakuda or Weapon Proficiency.

What's important here is that until Expert it requires a conscious effort to activate and maintain just like a Shinigami's Shikai, but no mastery is required to fully capitalize on the Spirit-for-melee effect that Shunko is intended to provide. Further development of the skill will take you into Kiriko and Kasumi territory where the Shunko itself becomes the weapon. There will be an "official" explanation as to how Shunko works based, as with most other skills, in canon, but just as with all other skills you'll be allowed to tweak the underlying theme of it so long as you don't expect said tweaks to bring with them any practical benefits. And of course, what your character chooses to make of the leap to Grandmaster is entirely up to them.

It's not "official" until it's official, but I think it's fairly safe to say that this is a solid enough starting point for those of you who want to wield Shunko; it's pretty unfair to dangle it over your heads like this and now, at least, you know roughly what you'll be dealing with.