So for those who don't know. In FF7R Tifa would build ATB using regular combos to activate Unbridled Strength. This allowed her access to more hits in her combo and a 2nd tier finishing move. I forget the names. Using this ability again would get you your max amount of combos and a tier 3 finishing move. Once you used any finishing move aside from the tier 1 finishing move you would go down one level of Unbridled Strength. Loosing access to your extra combos and that tiers relative finishing move.
My suggestion would be to apply this. Sort of.
Let's take MNK's Demolish combo. Normally you do Bootshine > True Strike > Demolish.
In my rework, or remake, doing this combo; basically ending any combo with Demolish or Snap Punch; would put you in a heightened state allowing you access to a new finishing move, and 1 new regular move. (Note you wouldn't have a tier 1 finishing move. You'll see why.)
Next you then perform another combo adding the 1 new move. So Bootshine > True Strike > Demolish > New move 1. Performing New Move 1 puts you into an even heightened state allowing you a tier 2 finishing move and 1 new regular move.
Performing the full combo, Bootshine > True Strike > Demolish > New Move 1 > New Move 2 as an ex, grants you your final tier 3 finishing move.
Now. What do you do with all these finishing moves. You use them back to back. Starting with Tier 3, which drops your heightened state down a peg. So it would look like this. Finishing move 3 > Finishing move 2 > Finishing move 1. Finishing move 3 does more damage on it's own than Finishing move 2 or 1, however performing the combo makes Finishing move 2 and then 1 stronger so that it does more damage than just building and spamming Finishing move 3.
Please note none of these things have timers so you would never loose your Heightened state or Finishing moves during boss transitions.
Now lets work in Chakra. In the lore there are 7 Light chakras and 7 Dark chakras. I think there should be 2 chakra gauges, one for light and one for dark. You build light chakra every time you finish a combo. So Demolish / Snap Punch, New Move 1, and New Move 2. You get a dark chakra when ever you do a finishing move. This would grant 6 light and 3 dark per combo.
So how do you go past this. I suggest an equivalent to Drg's Life of the Dragon. Where upon performing a full finish combo, So Finishing Move 3 > 2 > 1, you would enter a zen state, making auto attacks every second, ala Hundred fists like the NPC uses and for FF11 fans, as well as allowing you access to some Nastrond or Stardiver like move, probably not both tho. This would have a timer and run out.
The reason I suggest 1 full finish combo and not 2 is because of how long it takes to build to that finish combo. That's 12 GCDs + the 3 finish combo GCDs. 15 GCDs in total. Perhaps that's to long for party burst windows, or perhaps you should leave single player gameplay in single player games lol. But anyway let me know what you think. Have a great day
Edit 1: To prevent button bloat as pointed out below, you would need different buttons to become different buttons much the same way RDM, DNC, and NIN would function.
I would also like to say I would change Fist of Fire, Earth, and Wind in this rework. I would have you learn them all at once upon becoming a MNK. What they do however would function like MCN Clean Shot, Split Shot, and Slug Shot mastery... sort of. The stances you currently use; Coeurl, Raptor, and Opo-opo; would become Fist of Earth, Wind, and Fire respectively. Maybe giving a small 10 potency increase. Not much because at later levels these would cause some of your moves to become new moves. Like say Fist of Earth causes Dragon Kick to become Six Sided Star, without the global cool down nonsense, (used as an example since both are kicking animations.)
You could even tie these to the moves you get from a Heightened state. So Upon Completing Bootshine > True Strike > Demolish, for example, Dragon Kick now upgrades into Six Sided Star, again just an example, and acts as the 4th skill in your weaponskill chain. This would reduce how many GCDs you would need to get to your Finishing Move Combo cause you now do Bootshine / Dragon kick > True Strike / Twin Snakes > Demolish / Snap Punch > New Move 1 > New Move 2 > Finishing Move 3 > FM 2 > FM 1.