Packaging for a new Rey action figure that seems to confirm that Finn carrying the Skywalker saber is misdirection:
I don’t think this rules out Finn being Force-sensitve or becoming a Jedi; I’d guess he’d probably need to have some aptitude to be able to make any use at all of a lightsaber. On the flip side of that, though, I’ve seen a lot of people elsewhere on the internet (who presumably haven’t been following this as closely as us) despairing about Finn wielding the saber because they’re convinced it means Rey won’t be a Jedi. It sounds like a ridiculous idea because everything that’s been revealed points to her being the main focus of the film, but it’s a thing people actually seem to be worried about.
Also, some of Ralph McQuarrie’s original concept art for Star Wars had stormtroopers wielding lightsabers. So, perhaps, in the world of TFA stormtroopers have been trained in the ways of that ancient weapon.