#2 Lionel Messi

Now, now, pitchforks down, please.
Lionel Messi is Barcelona... and it would be unthinkable if the great man were to waddle over to the Bernabeu and turn out in Madrid white, but then stranger things have happened in the mad world of football, haven't they?
Ronaldo (O fenômeno) did not want to leave - but he left to Inter, and ended up at Madrid. Neymar did not want to leave - but he left to PSG, and may well end up at Madrid. Luis Figo did not want to leave, and at the time he was as much a fabric of Barca as Messi is now - but he did, and you know where he left to.
Despite Josep Bartomeu's best PR-campaigns, the truth of the matter is that Lionel Andres Messi has still not signed a new contract at Barcelona, and will be free to sign a pre-contract with any potential suitors come January.
For Madrid, though, as much as the joy of the greatest F*** you in history would give them, the next option would be much more sensible thinking purely on long-term strategy.