It really seems like a no-brainer fight to make, and a more interesting one, on paper, than meets the eye, but ONE has always been averse to co-promotion, they've never done it, and Chatri is on record stating that the UFC, who doesn't co-promote (after their experience with PRIDE and one off with Mayweather), is the only company he'd do it with. They've had agreements of sorts with Pancrase and Shooto, but it was those companies champs and top prospects fighting in ONE, to my knowledge they were like LFA and Cage Warriors are for the UFC, feeders.
Reug is now probably trapped in a ONE contract in the way UFC champs are locked into theirs (or worse), so by the time these two would ever be available to square off, they'd both be old as shit. Francis would probably be long retired from MMA. I hope ONE throws a curveball and pursues that matchup, co-promotes with PFL in Saudia Arabia or something, where many of their investors are in close proximity. That benefits both sides, to a degree, bc SA can pay for it, and if ONE has any success in a PFL/Bellator vs ONE card, that shows their investors that they're using their money towards an organization with some substance. I don't think Chatri has the balls to risk it, the gamble is too big. Conclusion, the fight will, unfortunately, never happen.