I don't think they should work on another branch off from FFXIII. I don't even think they should be making another Final Fantasy game for a while. Final Fantasy 13 was a lot of problems from the entire JRPG genre rolled up into one with a few more added. Stunningly bad story, cut-scenes that drag on with little sense as to why it's there, paint-by-numbers characters, ineffectual protagonists(I'm looking at you Hope), fortune cookie style life lessons, arbitrary padding, and to top it all off, graphical fidelity over overall quality of the product. What they also added to it was the stripping of the open-world exploration that Final Fantasy was at once proud to tout.
To me, FF6 was one of the last truly good Final Fantasy games. The game had charm, it wasn't afraid to take shots at itself and the characters were overall fun or interesting to listen to. What I think they should do is shelve the franchise for a while to give themselves perspective and give more attention to the other franchises that they're just sitting on. I mean it's kind of hard to truly make a great product if they insist on forcing themselves to make another without stopping and taking a breather. Like what they say, you can't chop down a tree with a dull axe.