Why not give Kaiten effect of being able to instant cast Iaijutsu?

[Shurrikhan]

The way I interpret this? is having Iaijutsu be locked behind Kaiten.

Interesting... would definitely make our skill-kit more punishing then the original Kaiten similar to the Rabbit-head of shame for Ninja.

I don't see how skill A making GCD-length skill B instant cast makes the nonetheless sub-GCD skill B "locked behind" skill A. That would only be situationally true of skills whose uptime cost is reduced by reduction to cast time, such as on RDM, so that they would be balanced around the shortened uptime cost. Here, though, the uptime cost is the same in either case.

But, let's try that out:

If Kaiten's to therefore go from almost never an option to truly never an option, then you're left only with a bloated Iaijutsu.
At that point you may as well just slap the 20 Kenki cost onto Iaijutsu itself, do the swish, then charge for .6 (post-trait) to 1.0 (pre-trait) seconds more, and use it there. Alas, that, too, costs fun, flavor, nuance, and flexibility alike.

I don't see any solution here short of the skill being of variable gauge-efficiency (such as per having a fixed percent damage bonus), costing less Kenki than one makes normally but near enough to feel like a significant margin consideration, and ultimately still being optional. That requires, in a nutshell, pretty much exactly the old Kaiten design, as Celest said above.