This isn't quite worth doing a full mock-up for, but just imagine that rigid combos have been, in most important ways, removed, in favor of more general "flows" and MP consumption. In place of those rigid combos you instead have [1] a larger variety of additional effects by which skills may benefit more than a single other skill without necessarily providing a universal buff (+Haste, +Damage, etc.) either, and [2] broader "flows" akin to combos, whereby a skill may reduce the MP cost of others of like type and/or part of the prior skill's potency can be duplicated onto that higher-order skill of like type.
In this way, you still have "finishers" of a sort, but at more variable length, and with far more variable pathing, and every job has significant access to bursting vs. conserving resource even without oGCD banking. In the above case of Samurai, let's say that Gekko, Kasha, Yukikaze, Oka, and Mangetsu always deal their full power, but at very high (unsustainable) cost without prior actions. In this way, MP itself can offer everything Meikyo Shisui did, but far, far more flexibly.
Other logical compliments to such a revamp include skills having Stagger effects (with a more sophisticated CC or displacement system backing it), interactions with enemy Armor, all skills doing as much AoE as they appear to (if your skill cleaves from left to right, even if previously categorized as "single-target", it now cleaves anything that its animation would go through), etc.
You simply give a third opening attack, a third skill post-Fuga/Fuko, and allow Shifu and Jinpu each three possible outcomes (e.g., can funnel into Oka, Kasha, or some new skill, or Mangetsu, Gekko, or some new skill, respectively). In this way, 3 buttons covers every viable rotational melee option at every GCD. Iaijutsu, chaining into Tsubame Gaeshi (whose CD surrounds the current Iaijutsu and remains usable until your next Sen generated) would be your fourth and final GCD key. (Enpi would be a passive extending the range of your ST line at %damage cost and chargeable over time and by Yaten to reduce that potency loss, thereby retaining control, timing anchors, and optimization (for ultimately more skill expression)... without costing a single button.