Because of the long term bickering between Rollins and Kane, it's probably going to either one of them. My pick would be Rollins.
Kane is firmly entrenched as an Authority stooge, but then again with the new stipulation at HIAC, you never know. I'm not sure I like Kane as a face, his whole demeanor just screams heel.
Rollins on the other hand while he's been a great heel, is a much better good guy. His fast paced, high flying style of wrestling fits the good guy image, and I have no doubt if they were to turn him, we'd see a much stronger Rollins in the ring.
The only issue with turning Rollins is it leaves a vacancy for the top heel position. They still have Rusev, Wyatt and Owens to fall back on, but my fear is that Sheamus will take over Rollins spot. He would be the perfect new Authority heel figure as HHH would now be almost in control of the MITB suitcase, again, and the WHC belt when Sheamus cashes in.