I'm usually not the guy who criticizes something and says how he can make it better, but I feel they could have went a different (and personally I think a better) direction with their combat.  A lot of this may sound reminiscent of FF X and that's because I liked FF X and think that maybe they could had built off of that system rather than starting from scratch.  Let me know what you think of my proposed changes and submit your own.
 
1) Characters have only one class that fit them most.  Lightning = Commando and Snow = Sentinel, and such forth.  They can unlock different classes later on, but most of the game, every one is one class.
 
2) Hot swapping characters rather than classes a la FF X.  However, unlike FF X and to keep with the spirit of the game, you switch by the paradigm system, creating 6 three man teams to choose from and swapping teams mid-game.  Thus, you can't switch individual characters out, but rather teams of characters.
 
3) Switch between party leaders with the L2 // R2 buttons (ps3 mapping of course).  Seriously though, this should be in there regardless.  I don't think anything is mapped to those buttons already.
 
4)  Move Party Leader with the left analog stick.  Menu choices will be selected by using the directional pad.
 
5)  "Square" (or "X") will auto set you combat moves that turn and automatically pick a target.  You can cancel it at any time with the "O" button or use the "Triangle" button in the same manner it is now as well.
 
 Anyway, I don't see why it couldn't be done that way, and I think it would've worked better.

I haven't played FFXIII yet (I've played all of the others though), but the hot swapping of whole teams doesn't sound like it would really add much to the gameplay. Swapping abilities/weapons/characters would be much more useful, if it already does one of those, then its probably for the best.
 
#4 sounds irrelevant but I don't know how it currently controls. If its D-pad and analog do both then that's much better, and usually how they do it anyways.
 
Auto-setting combat moves for a turn sounds like a good idea, I always wanted that kind of feature in the previous FF games, but it really depends on how the rest of the gameplay works out.. it could make the game feel too automated and that usually isn't a good thing.\
 
Personally, I liked the direction the Final Fantasy series was going with the combat in 12, but I really wish they would get a clue and add in cooperative gameplay. FFXII was the perfect game for co-op, not including it was a mistake I think.. They really only would have to do co-op on the battlefield.. and in 12 that was segmented from the cities, so it would work out nicely. Giving both players the ability to run around the city and fetch quests, buy items, etc would have made it all the better.
 
It's so strange how Chrono Trigger still has one of the best combat systems in any RPG ever...developed by Squaresoft even and they haven't been able to build a more dynamic and fun combat system since?! Very weird...

I don't know, I am really loving the speed, immediacy, and strategy that this new system has. 
 
It's like playing chess only real-time and overdosed on caffeine

I love the battle system, the AI for your team mates is great as they always seem to do the right move.

I like the idea you have number 2, but outside of that I can't really see how to improve the battle system. I love it, so many options, it's very fast paced and has a different feel and strategy needed than other FF games. Loving it so far.

I much prefer swapping classes than characters, especially considering how split up most of them are throughout the game. I am glad they went this route instead. 
 
However, I must agree on one point, the only flaw I feel is in the combat system is not being able to move characters to some degree. It can get annoying when your Sen decides to tank stuff right beside characters like Hope and Vanille who do not have a lot of HP take aoe damage that could of been avoided otherwise. Honestly though I feel this could be solved by simply programing the AI to move when attempting to tank mobs.  
 
Also a nitpicky thing really but I hate it when the class change animation goes on an individual character basis, it should always be every character at once (which is usually is, so whatever).