I'll just come out and say it. Finn is Han's son. See, scoundrel that he is, Han has an affair with Lando's wife, producing Finn. Lando's furious and later sets up Han by turning him over to the Empire before later feeling guilty about it and saving him. But in order to save his marriage, which fails anyway, the couple give the kid up, who isn't known about by Han, to the Empire before Han is frozen in carbonite. Lando, still sore and wanting to get even w/ Han, not only turns Han over to the Empire, but also flirts with Han's girl Leah.
Now, I know. I know. There's some puzzle out there that has a package that says Lando is Finn's dad, but this is actually reported as a mistake. Besides, since Han had left, Lando would have been the male figure in his life until Finn was handed over to the empire.
Furthermore, if you look at personality traits, is Finn anything like the suave, smooth, ladies man that Lando is? Heck no! He's more like that bumbling, con artist, nerf herder Han. Both start out as non-good guys who, despite not wanting to express a heroic streak, end up turning away from their old ways to help an undermanned group of heroes save the galaxy from a dark menace, and become heroes.... reluctant heroes, but heroes none the less.
Han is Finn's dad. That's the twist. And one no one will see coming.