I'm kind of confused what would make the P4A system adapted into a simple team system ala MvC complicated.
Let's say a player forms a team of Yu, Yosuke and Chie. Yu is on point, with Yosuke as assist 1 and Chie as assist 2. Two buttons would have to be added to the game, making it a 6-button fighter.
The battle starts as it usually does in P4A with P1 controlling Yu. Everything the same, except for now if you press the A1 button, Yosukeoff-screenwill perform his 5D. If A2 is pressed, Chie would perform her 214C.
For balance purposes and to not make things overwhelming, breaking an assist Persona could just take away a Persona card off of the point character, which would persist if an assist character was switched to point. If not, all the assists could just be character based, making Yosuke's j.236C and Chie's 236C.
Silky brought up snap-back in relation to a Persona. If it would have to have it, it would just work the same as if an assist character is hit with it in MvC as a Persona character in this hypothetical P4A: nothing. You'd waste meter and, here, the Persona would get broken.
Aside from that, the rest would just be like the active team system of MvC. It would obviously be different fundamentally than P4A is now, though. I prefer as it is now.