Cardio?
The first thing that comes to mind is his Cardio as he clearly looked the more exhausted fighter in the 5th round. But that can be attributed more to the elbow he took in the 4th. He was looking pretty nimble in the 4th round up until that point. Also, More than the improving his Cardio I think Gus needs to work on avoiding those spinning back kicks. He took a few right to the abdomen and immediately had to take deep long breaths. I could be wrong but I think his cardio was at the same level as Jones'. He just needs to find a way to not get hit cleanly with kicks to the abdomen.
Aggression?
With a chin like that , I think Gus could afford being a little more aggressive. Unlike other fighters he does not have to take the risk of overcommiting in order to close the distance to land something as he has the reach that matches Jones'. If there is a perfect candidate for upping the aggression against Jones , its Gus. Not saying Jones' chin is weak, but Gus' is definitely proven.
Though, Both Jones and Gus were near perfect IMO. I don't see room for any big improvement. Just little ones that could tilt the fight a little.
So, If you were Gus' trainer/coach which improvements would you see as the crucial ones?