Gotze will definitely be world class (he's not far off it already and is still a teenager!) but only an obscene offer would pry him away from Dortmund now. But then again he might be worth the investment as I agree, I think he could become one of the top three players in the world.
But realistically Bale or Di Maria. Di Maria still hasn't signed an improved deal and was annoyed that Real wouldn't improve his wages so a big offer could tempt him away.
Lavezzi would also be a good signing but the Italian media are convinced he is 100% PSG bound. Belhanda sounds like he could be worth the risk too but he has known disciplinary problems and might need to mature a little first.