Being a HW, typically he has a few more years in him (injuries aside).
There is no reason why he can't have been working on his skill sets in all of this time off, as he was injured but not paralysed.
So, if his skills are increased and we are mostly left only with his need to get that cardio tank back up, then that is very doable.
Unless he is injured again, then I don't see why we can't see at least a decent 2 years back in the mix.
I think that Cain is very capable of delivering a one sided beat down to Stipe, ala Dos Santos style and the same against most of the HW pool, if not all.
The only HWs that are considerably younger are not really the best performers.
So, I still think that Cain can and will very likely be champion again if he returns this year, even for one fight and stays healthy. Of course, it is all dependent on that.