It cannot possibly be Mace Windu, because Mace died at the end of Clone wars, at the year 32BBY, and the events of TFA happen 66 years after that, meaning that Finn would have to be at least 65 years old in the film (barring artificial insemination after father's death).
He clearly looks WAY younger than 65.
While this is more of a speculation than direct evidence, there is a SMALL chance that Finn's father is... drum roll:
The basis for this theory is the fact that, according to New Disney canon (specifically, Marvel's 2015 Star Wars comics), Sana Starros - a dark skinned woman - showed up at some point before and some point after Battle of Yavin and claimed that she was
Han Solo's wife to Princess Leia Organa (which Han denied!)
It is possible (though in my opinion not very likely) that this was a hook which Disney used to later reveal that he was Finn's father, his mother being Sana Starros.