My guess is near-zero at his full salary charge, but significantly more likely if NYCFC covers a significant part of the costs.
In a sport where the domestic league is essentially they universe of players, I could imagine an expansion team taking a guy like Mix, even if he is overpriced relative to performance because there aren't a lot of other alternatives. But, in soccer where the potential player pool is global, I can't see why they would absorb the full $750k.
If I had to guess, a return to Europe still feels a lot more feasible, though the unknowable piece is whether he personally wants to do it.