The only thing that concerns me with ponga at fullback as early as 2017 potentially, are the kick returns. He's not massive yet and he could be an easy target running into the big boys 1 out.
Still remember Sam Tomkins with us, getting dominated on every return.
Ponga is a freak of nature, he hits it up the middle straight into the props and often makes 5-10m after contact, despite his lack of size (not just in the u20s, saw this happen in a trial against most of the Broncos first grade squad). I think he'll be fine, especially with another off season to gain some weight. He had a brain infection last year, was bedridden for months and he lost a lot of weight. He's extremely tough.
For those who haven't seen him play, he's not a winger (he hadn't played there before he did in the NRL), I think he'll move to the halves eventually. He has everything, including the mentality to succeed. Injury is the only thing that will stop him becoming one of the best in the game IMO.
I'm a Cowboys fan and although it's disappointing to lose him, I'll follow him throughout his career. I want him to shine, he's the type of player who will sell the game to new fans.