I think that Battle Regimens will be changing a bit, implicitly, through the rest of the battle system changing. I'm still trying to imagine how it's going to work with the stamina bar gone.

That said, I have a few suggestions that seem like they would be fairly minor (small enough to implemented soon?) yet may have a big impact.

Allow Auto-attacking to continue while a Battle Regimen is queued up.
I think that one of the biggest deterrents to performing a Battle Regimen is waiting around while the rest of your party gets their actions queued up. For example, I queue Twisting Vice, and sit there for 6 seconds while my duoing partner activate Ferocity, Bloodletter, etc, powering up his weaponskill before adding it to the chain. It makes sense for him to power up, but ... well... it just feels boring sitting there doing nothing while it happens. Better timing / communication, blah blah, I still think that keeping AA going while the BR is chained would be nice.

Better feedback as to the results of a Battle Regimen.
The four battle regimen combos that inflict status effect -- the on-screen feedback that something has happened is FAAAAR too weak. Either trying to catch the name of the Regimen in the torrent of battle text, or noticing the tiny little new status effect icon on the enemy. Compare that to the "Light Party" / "Full Party" effects. Have some flashier on the screen to reward players for completing the regimen and give more clear feedback that the regimen effect took place.

As to the "power-up" regimens. Provide some clear on-screen feedback as to how the regimen is affecting your attack. If we correctly chain four weaponskills / spells from different classes so that my weaponskill is powered up by +250%, PUT A BIG +250% IN NEON LETTERS ABOVE MY CHARACTER'S HEAD. Some fancy and classy, ala FFVII-IX battle effects. The point is to visually reward the player for the teamwork, as well as to provide solid feedback as to what the regimen did. My original though on this was the big-scale, five minute summoning type quasi-cutscenes from earlier FF titles, but I realized that it need to not be so time-consuming to make it a chore to wade through the animations.

That is all for now ...