Not being a dick guys, but we've had plenty of wonderkids that didn't live up to their potential threads. OP specifically states some criteria and even lists Ravel Morrison as an example of who not to list, yet some people are specifically listing him. Maybe actually read OP's post?
A lot of people just didn't read OP
The youngster Euro winning squads are full of guys like that. In France we had the "infamous" 1987 generation led by Benzema, Nasri, Ben Arfa and Menez... Their squad had some guys that fitted the OP. Two standout example :
Ahmed Yahiaoui, who I think was one of Nasri best friend growing up, he was touted as a gem signing his first professional contract with Marseille, people in circles said he was "the best" between him and Nasri IIRC... Two years later (19yo) he plays one game in Switzerland league and disappears not notifying his club and then proceed to go from french amateur club to club (and some algerians clubs also) and retired at 28 years old playing in amateur leagues.
Stéphane Marseille, the urban legend of Champagne Ardennes.
I was born and raised in that region so I will go a bit deeper on this one.
He started to play with Reims team at 17 years old in 2nd division as a very very rare (at the time) kid playing for the hometown and was part of the aforementioned hyper hyped french NT. Everybody talked about him in U13-U17 football circa 2004/2005 in the region. Long story short, by the time he was 20 he was playing at amateur level in CFA (4th division), then at 21 he was playing in DH (6th division), at 24 he was playing in PH (8th division).
By 25 he stopped football to concentrate on his wife and his kid. He tried to replay football at 28 only to have an achilles tendon rupture. He now works as a seller/cashier at Intersports (french equivalent to Sports Direct) in a small city.
What happened was that he left Reims because he couldn't agree on a professional contract (he said they wanted him to abandon trying to pass french equivalent to A Levels and didn't agree), went to Nancy in Ligue 1 and was about to start playing regularly with the main team but cameback to Reims for a girl (not his current wifey...) only to go back to the reserves and he was treated (I think rightfully) like a "everybody", wasn't a starter... and felt he fecked up then as the time passes went to lower and lower divisions.