4. Kylian Mbappe
Kylian Mbappé already has 68 Ligue 1 appearances to his name, two Ligue 1 titles, a domestic treble, and has just won both the World Cup AND it's best young player award. All of this has incredibly happened before his 20th birthday, making him not only the best youngster in the game, but one of the best players in the world, period.
All these credentials inevitably put the second most expensive player in history on Madrid's map, and though he would cost far more than the money Real received for Ronaldo, the Frenchmen could potentially lead the Spanish giants to glory for the next 10-15 years at least.
While it may be a problem that Mbappe only technically signed for PSG permanently this summer (after playing his first season in Paris on loan from Monaco), the solution to this lies with Benik Afobe. The striker signed a permanent deal with Wolves on 1 July only to be sold to Stoke City less than two weeks later - it sets a precedent.