So there's been multiple theories surrounding Rey's parents and Kylo Ren's parents (although that's been confirmed if you've seen the movie) .
One character we can't forget is Finn. He and Han Solo gave the audience a lot of laughs throughout the film. Despite what a funny and good hearted guy Finn is, his background is shrouded in mystery. Where did "FN-2187" come from you may ask? I have two theories.
1. Lando Calrissian.
If you've ever seen The Empire Strikes Back, you know that Lando is an important character. He and Han Solo were buddies with a long history together (despite his betrayal and turning his friends over to Vader & friends) Finn and Lando share many similar traits, specifically their sense of humor. This would also explain how Finn is around the same age as Rey & Kylo Ren.
2. Mace Windu.
Okay, so obviously Finn cannot be Windu's direct offspring, that would make Finn around the same age as Han or Luke. It'd have to be papa Jackson's grandson. This would also provide a good plot line in the sequal. Perhaps Finn meets his father who isn't in contact with the force and never wanted to live up to Mace's legend? and maybe Finns father even has Windu's purple lightsaber? After all, this would give Finn another reason to fight the Sith after Windu was blasted out of a skyscraper by the hands of Emporer Palpatine (after Anakin betrayed him)
How cool would it be so see Finn wield Windu's lightsaber?!?