He won't go to the World Cup, no matter what. Especially not if he leaves. Del Bosque is all about squad harmony which is why he keeps nominating Torres for every tournament and leaves the likes of Negredo out despite the latter performing better than the former for quite some time now. Del Bosque won't include a player who left Barcelona for his own glory when the majority of his squad is made up of Barcelona players. Imagine Thiago and his father causing disarray by demanding playing time during the World Cup campaign once he's in the squad as they are doing now at club level.
Ultimately: Who would make way for him anyway? Here is the Confed Cup squad minus Xabi Alonso:
Casillas (Real Madrid), Reina (Liverpool), Valdes (Barcelona); Alba (Barcelona), Albiol (Real Madrid), Arbeloa (Real Madrid), Azpilicueta (Chelsea), Monreal (Arsenal), Pique (Barcelona), Ramos (Real Madrid); Busquets (Barcelona), Cazorla (Arsenal), Fabregas (Barcelona), Iniesta (Barcelona), Martinez (Bayern Munich), Mata (Chelsea), Navas (Sevilla), Pedro (Barcelona), Silva (Manchester City) Xavi (Barcelona); Torres (Chelsea), Soldado (Valencia), Villa (Barcelona)
3 goalkeepers, 3 strikers, 2 CBs + 1 CB backup, 1 LB + 1 LB backup, 1 RB + 1 RB backup. <- This is set no matter what.
That leaves us with 10 players for the midfield and wide positions. Let's see:
Xavi, Iniesta, Busquets, Alonso, Silva, Fabregas. Those 6 names are set in stone because they are either starters or close to being starters (like Fabregas who plays a lot despite not being a 100 % starter every time).
Navas will be included as well because he offers tactical variation.
Pedro as well. He's their freaking top scorer this year and deserves it for his goal against France in the qualifiers alone if nothing else.
So that leaves us with Mata and Cazorla. Both among the top players in the Premier League. Thiago would have to BLOW them away with his performances next season. Not just outperform them but BLOW them away to get their spot. Not going to happen.
And I haven't even considered Javi Martinez and Isco yet. Both would be more useful for del Bosque's team. Martinez because he can also play as a centerback if needed. Isco because he can play all 4 front positions (wide left, wide right, central, false 9). Thiago can only play centrally.