Didn't abrams want to do that? I dont think it fixed a lot of the issues, but it would have turned out better if they did.
Also, call me crazy, but I actually hate the idea of finn being a force guy. I think it takes away from the main protagonist having those abilities, and honestly don't see a need for him to have it to be important. The problem was he became an afterthought in 8, not that he didn't have those powers.
There was rumours/leaks JJ wanted to split it into two. Although there were stacks of crazy rumours, but this one is believable.
Finn being force sensitive would help explain him breaking free from his brain washing. If you look at TFA with a charitable lens, something many people are not willing to do. There's the possibility of many interesting stories being set up.
In the final Kylo vs Rey fight. Finn takes a stoic pose to fight Kylo after finding Rey unconscious.
When Rey takes up the blade after Finn is injured, she's struggling but then she has her, "let the force moment." Afterwards she is full of rage and anger. Suggesting she's tapped into the dark side of the force. Kylo's reaction is one of being scared. Rey completely mirrors Luke tapping into his anger in the Throne Room, but she never pulls back from it.
The seed's are planted there for Rey to go dark. The former Storm Trooper turned Jedi to be in the light. People picked up on this at the time. It's easy to say JJ didn't know Star Wars, the Jedi but if you assume he wasn't completely unaware. There's definitely groundwork being laid.
In TROS, JJ clearly is dumping a whole bunch of ideas for his trilogy through out it. Almost like he's saying, "Don't blame me for this trilogy, I had a whole bunch of idea. Blame the other guy."So it's like, "Rey could have gone dark, here's a picture of that. Finn is force sensitive." It's probably not coming completely out of no where.