I came across a fantastic article, surprisingly written by the folks over at ESPN on significant transfer fees that could have been.  I recommend reading it Source: 
http://soccernet.espn.go.com/columns/story/\_/id/1125402/first-xi:-the-deals-that-could-have-been?cc=5901

But it made me think that this would be a great topic of discussion. There are many more than the ones listed in the article and I thought everyone could maybe research a few from whatever team they follow or have an interest in.

I'll start. Obviously, I'll cover the Arsenal but feel free to add any I miss.
Take a look at this fucking list (some are mentioned in the article):

1- Diego Maradona (1978) - in his prime!
2- Johann Cruyff (1976)
3- Christiano Ronaldo (2003)
4- Zlatan Ibrahimovic (2002) - You need to read Zlatan's quotes about this (they're just too funny).
5- Yaya Toure (2003)
6- Didier Drogba (2002)
7- Petr Cech (2002)

I cannot wrap my head around some of these. Let's be clear these weren't based on speculation or loose associations. These were deals that were inches from being signed. Some fell through because of monetary reasons, some because of personality issues, while others were simply errors of judgement. In any case, no one can say for sure that that these players would have been just as good had they signed. Still, that doesn't mean we can't imagine "what could've been".

PS: IF YOU'RE GOING TO CHIP IN, MAKE SURE THEY WERE DEFINITELY CLOSE TO SIGNING. I DON'T A LIST OF WILD SPECULATIONS THAT HAD NO GROUNDING IN REALITY.

I guess no-one's interested :(

Signings that changed Barcelona's history and football too:

1. Ronaldinho 2003. Before he signed with Barcelona, the club was without soul losing all the competitions possible and the fans were blaming the barça's playing game. When Ronaldinho came to Barça he changed everything giving a heart to the team and impressed all the fans with his magic skills.
2. Messi 2000. Messi signed with barça at 13 years and he was doing it better with the years. In 2008 when Ronaldinho left the club everyone thought that Barça won't do anything but there came Messi and Guardiola and they have won 6 titles in one year. A thing that none did it before in football's history.
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   I could write more players but those are the only players I have seen playing and I thought that one day I'll say to my soon: "I have seen Messi and Ronaldinho playing football" and this way I won't do any speculation.

SAF wanted to sign XAVI in his youth

harry redknapp changed tottenham 

manchester was very close on signing Piqué after his loan

Xavi almost went to AC Milan in 1999