4. C-3PO's 'Phantom Limb' - Metal Gear Solid V: The Phantom Pain
Now this one is just plain cool. So the story goes that Metal Gear Solid-creator Hideo Kojima and Star Wars-resurrector JJ Abrams are good friends (seen here). The former was busy polishing off his latest masterpiece, MGS V: The Phantom Pain, right when JJ was still working on The Force Awakens.
Kojima reportedly visited JJ on-set, the two got to talking, and C-3PO's randomly red arm is reportedly a result of this; an immortal tribute to Hideo and the version of Big Boss seen in MGS V, who's also sporting a red (phantom) limb.
As a bonus fact, listen closely during the opening Jakku battle in the movie and you'll hear the "Red-Eyed Sand Alien" (as they're credited) utter the word, "Kojima" as BB-8 is rolling away - another nod to the cinematic game director.