On the subject of forgotten players you wrongly thought would become world class, I'll put forward Daniel Opare.
I watched him at the 2007 U17 World Cup with Ghana and he was an amazing attacking right back - getting up and down all game at pace, good on the ball, and making great contributions at both ends.
I was raving about him and he quickly joined Real Madrid and progressed to the Ghana national team. However he failed to establish himself with either and his career drifted for a number of years. I forgot all about him but, having checked, he seems to have had a couple of decent seasons in Belgium recently - although he's 30 now so really hasn't achieved anywhere near what I thought he might.