I personally think Graces F's battle system is the best in the series. They have really pretty much perfected it. I don't see a tales game with much more improvement in the system itself... I really hope they keep most elements of it.
The things I really liked about graces f' system comparing to Vesperia, Abyss, Symphonia, and so on:
1) Give you rewards for actually turning up the difficultly
2) Allow you to unlock all difficulties in one playthrough, which means you can experience almost everything in one play-though
3) Godly number of mystic artes
4) Combo activated MA, which rewards high # of hits
5) Switch characters on the fly without cycling, this is important and open up a lot of single player combo opportunities. I believe this is the first in the series to do so
6) Multiple way to learn/earn artes
7) If you are good at the battle system, you can master the titles really quickly, very few/almost no grindings needed.
8) Varieties of optional bosses
I can go on and on..
Personally think Graces F is the pinnacle of JRPG battle system/mechanics design. This is it, there will be no better ones.