All Ninja really needs is something extra in its single target filler. Like more ways to access Assassinate? I could see Dream within a Dream, Hellfrog/Bhava, and the combo finishers directly interact with a gauge that’s effectively charges for it. Then just upgrade it to a slightly more shadowy action that has an AoE component. It might need a little more Ninki generation to work, but I can’t see changing much beyond Mug’s cooldown, maybe giving it charges too.

Only other things I can think of is overcap protection on Mudras (Max 3 charges during combat, 2 otherwise. Hide can’t bypass this) and Meditate for Huton/Assassinate.