What I'd want is IQ better than the 5Dmk3 cropped 1.6x (i.e., in focal-length-limited situations) even at (moderately) low light with a good enough lens. That is, the "crop factor advantage" would have to be actually realized. I don't think that's an impossible wish - FF bodies would still beat it hands down in low light when there's no need to crop.
Besides that, I'd want better movement-tracking AF (at least 5Dmk3 level, preferably better) and speed (I could live with 8 fps, but I'd want a big buffer - that's my only real gripe with the 5Dmk3 as compared to the 7D).
Otherwise I don't care much - GPS, WiFi, dual card slots would be nice but not important, video I don't care about.