Finn is originally part of the new order, if you're questioning a black man being a storm trooper let me learn you real quick.
The cloning facility on Kamino was destroyed before episode 4 ever took place so absolutely no new clones were made past this point. Secondly, clones grow at an accelerated pace, by the time they're 5 they're battle ready (I.e. episode 2) by the time they're 15 they're no longer mission capable. Chances of clones remaining in the original trilogy is impossible, let alone the upcoming trilogy. Sorry, that bit of info needed to be said, got your back Boyega.
Jumps sides to the rebels and is depicted wielding a lightsaber. Rumored to be Luke's. If you remember Darth Vader and Luke's duel in cloud city you will remember that was when Luke lost this blue lightsaber and subsequently his hand as well. However, thus lightsaber was not of Luke's construction but that of Anakins (given to Luke by obi wan in A New Hope). Luke's lightsaber is actually green, so that is not his which would bust the theory behind Finn and Luke broing out with their lightsabers out. If, that is in fact anakins lightsaber, then this could get interesting.
There is a theory going around that this new Dark Lord is a big fan of Vaders and he is looking for Vader relics. (I will not go into full detail on this theory because it is not my own, i don't want to butcher it, and it's a lengthy well written article itself that I recommend reading).
So with that in mind here are some Finn theories in reference to this mystery lightsaber.
1. Finn is the Dark Lords apprentice and he has found Anakins lightsaber, that his master was searching for.
2. Finn really is a rebel and he just lucked upon this gem or he knows it's value and is protecting the lightsaber from the Dark Lord.
I don't see either of these theories being real, but it's food for thought.
Now, the actually juicey portion of this article and theory; Rey, General Hux and Kylo Ren.
First off, Rey:
Shortly after episode 6 Leia and Han were both captured, separately, and kept apart. The star destroyer Leia was on crashed on a desert planet....jakku? Maybe? Hmm... Anyways she had the twins and her and a Mof raised one of the two... Rey. The other was taken by another mof, raised and trained under the influence of the Dark side.
This is where it gets interesting. Who is the other twin?
Is it Hux? Arguably too old compared to Rey, but it's possible. He was raised by a Mof and clearly would fall on the emperial side because of this.
But if he was trained in the ways of the Dark Side of the force... Why would he be a general? Why wouldn't he be a Dark Jedi?
This is where Kylo Ren comes into play. What if this is the other twin? Being a child of Leia, with family like Luke and Anakin, it is very apparent that the two children of Leia would be very strong in the force. And even in the books, which are no longer cannon (sad day), one of the kids struggles with the darker ways. Who wouldn't? The dark side is awesome. They use the full power of the force and don't get politically correct about the less humane parts like the Jedi do.
Assuming Kylo is the other Solo here's my theory.
In drops Rey SOLO, strong in the force joins the good cause like her family did. Runs into Finn, who just turned in his storm trooper armor. Finn just happens to know some of the more important plans of the enemy. They team up, search out Anakins lightsaber and somewhere along the way Finn learns of his gift. Kylo, seeks out this dynamic duo in order to claim Vaders old saber for himself. Kylo and Rey are unaware of their family connection, hopefully they won't kiss like their mom and uncle... The world, nay, the galaxy doesn't need another set of kissing twins. Meanwhile the original four are probably looking to reunite after 20 years (star wars timeline) to make a badass movie and appease us fans that have missed these guys for the last 40 something years. In pops General Hux, most likely alligned with Kylos to fulfill his own personal agenda. But wait? Why would Hux care about Kylo? Maybe because Hux is the one who raised him the last 20 years. Sealing Kylos allegiance to Hux and his pursuit to crush the rebels. Now Hux can use this Dark Pawn for whatever he wants, he owns him now.
Again this is me just jotting down thoughts in my head, could be legit could be completely off, that's the beauty of a theory.
Star Wars Episode 7: The Force Awakens opens December 17th. May the force be with you!