Sounds like he has a future as a pretty interesting power forward.
He's similar to Anthony Randolph in that respect. Randolph's issues have been mostly mental in nature, I think. So perhaps Giannis can avoid that path.
Perhaps Giannis will become something like Lamar Odom, only better defensively and not as skilled offensively.
On the other hand, he might just be another Austin Daye.
His ability to add weight to his frame and gain strength without losing too much agility or fluidity will probably determine his long term success.