Ngannou-Jones is a dream matchup ... but things heated up this week when Jon stopped by the TMZ office, and talked about the possibility of fighting Francis.

"There's a lot of fights at the heavyweight division that are always gonna be extremely appealing. Cain Velasquez, Francis Ngannou," Jones told us.

"That's a scary dude. But, that's why I'm in it. For it to be hard. I'm aware that the greater the challenge, that's what makes it great. The hard is what makes it great."

So, Jon's down ... question is, is Francis willing to put his body on the line against arguably the baddest dude on planet Earth?

We have our answer ... and it's a big hell yes.

"Personally, I mean, it would be a good fight, first of all for the fans, for me, yes," Ngannou told us.

"Even though Jon Jones is a light heavyweight, this guy has made his legacy, dominating the whole division. He's a big dude. I met him once, we almost, we are the same height, we are almost the same thing. I might just have a few pounds on him. That would be a legit fight."

Before a fight with Jones or anyone else (D.C., Stipe), Ngannou's facing badass former heavyweight champ Junior dos Santos at UFC on ESPN 3 this Saturday -- so he's got his hands full.

But, it's very clear that Francis -- who says he ALWAYS gets asked by MMA fans about fighting Jones -- is down to make this fight happen.