All right forum let's see some players who had potential but fate had other plans. Basically they failed to reach their initial hype. Let's see some players who fit the profile.

First off is David Bentley. Touted as the successor to David Beckham but now is literally a reserve player for Spurs. 

Jermaine Pennant!!! My god what a player he could have been :(

Hélder Postiga. Had an enormous potential while playing for Porto, but after having pushed the ball a couple of times for Spurs, the likes of Postiga turned quickly. He failed to adapt to the English style of play, only managing to score twice. As a result, his reputation as an upcoming star drastically decreased.

Ruben De la Red, he was a former Real Madrid player and as a youth was a great prospect. He spent a season on load at getafe and then discovered he had a heart condition forcing him to retire.

Paulo Ferreira - Chelsea

In the 2006-2007 season, he suffered from lack of confidence and lost his spot to Khalid Boulahrouz and to midfielder Geremi. Then he was described by Jose Mourinho as "a player who will never be man of the match but will always score 7/10 for his indivisual display.

@raimondo I remember De La Red. Poor guy was forced to retire. I think he had a few caps to his name as well. 

@koldimere yes he did. Part of the Spanish euro 2008 I believe. 
Squad

Patrick Viera! He signed for City but was benched and later on retired

@Chelsearox10 Idk if Veira would qualify as a "could have been". He is a legend in Arsenal my friend. Viera moved to City when he was 33. That's when players normally wane in midfield. So it was kinda predictable. Think of a player who failed to shine in his prime age. 

I thought Santa Cruz would be a good addition to our squad after his awesome spell at Blackburn but he didn't. Good to see him back though.

@koldi there a few midfielders who are still excellent in their late thirties like Pirlo and Xavi. But I do get your assumption. That's the conventional age for decline.

Anderson - Manchester United

Hmm...Anderson a flop? I thought he was decent enough but is that what most ManU fans think? Is he like the Denilson (Arsenal) of ManU? HA!

Ok, then I think yury zhirkov he only played a few games with us and ended up scoring bout 2 goals, we signed him from CSKA Moscow for 18 million but sold him to Anzhi for free

Well... Ronaldinho could still be amazing if he stopped partying...

Anderson and Macheda