It goes without saying that there would have to be some tweaks to the mechanics. If you have six party members you could easily add opponents that would require more than one SEN to survive phases of their attacks, ect... I would also see severely reducing the amount of spells SAB and SYN's could cast, since you have all the party members each one spell should probably only be able to cast by one party member. In essence it would make encounters more like MMO boss fights, with different phases and attack pasterns that you have to constantly adapt for.

I though the battle system of FFXIII was good, not great, but is the first game with AI party members that I have ever enjoyed playing. Now that I can accept that a game with AI party members could be fun I guess I am just playing around trying to make something even better

Once I caught on to the fact that the battle system was more flash than anything else and I had plenty of time to make most decisions I had to it's seemed kind of slow to me actually.
We appear to be talking about different things
I was more referring on the inability to track stuff like the exact damage your party was giving/taking, and other numerical details like that. For example, if I add +10 Strength it is virtually impossible to track the actual difference in damage I am dealing. That was to which I was referring.