The thing I missed most about the series was the open world map that you could explore by land, sea, or air, which we haven't had since IX. XII came close, but I want to pilot an airship damn it, not select a location on a menu. Bring that sense of exploration back (with a gradual broadening of the world, until by the end you can go anywhere you want), and that could be the start of something good.
So, I guess what I'm saying is: XII's open-world with X/XIII's combat (XIII really was just a glitzed up version of X, minus the character-swapping, which they should bring back), and exploration from I-IX. Oh, and no more "buy increasingly powerful swords" to fight enemies with. Bullshit, I shouldn't have to choose between a dozen or so swords and figure out which one I should take for the next ten minutes of fighting before it becomes obsolete and I have to get another sword. However, if they want to do the license system from XII, and have everyone be able to use whatever weapons you want (though with some characters having different affinities), that would be fine. The story and setting don't matter, just as long as the stories good (no wasting of five hours before getting to anything resembling a point, like XIII), and I can explore the world (though I would hope it's not too gaudy).