Hasn't started playing FM 2015, waiting for the full release. But I would assume that the three players you mentioned would have some potential at least. Diego Llorente feels like a future Arbeloa/Nacho kind of player, versataile and reliable. Apart from that Derik, Marcos Llorente, Alvaro Medran and Jaime Sanchez SHOULD be the biggest talents.
Medran was good for me first season then he kicked off as i refused him a contract,his wage was adequate for his role in my team. Marcos Llorente is very promising plays every cup game for me.