I've heard a few theories that Jason might be working w/ Merryweather as a Mercenary!
That is a vibe I can totally fuck with. However, there's one thing that's been lingering in my head...
The only real enemies that I know of for Merryweather (outside of killing people during their security contracts) is Trevor. It's the only real enemy that completely embarrassed them and made those mercenaries look like fools. Trevor killing a lot of their friends AND got away with it.
Merryweather wasn't very frightening in GTA 5. Yet, I suppose they weren't supposed to be. However, they did provide some of the more challenging shootouts...
But, what if we get a mission for these mercenaries to extract information from a captured target. And we enter a room just to see a heavily mutilated man in a chair. Beaten. Body parts missing. No eyes. Just horrifically disfigured. And as we get closer to this man, we realize that it is Ron from GTA 5. What seems like a simple "side distraction" mission, is an operation to brutally dismantle TPI Industries and perhaps even "deal with" Trevor. And I mean, deal with Trevor in such a way that would make the player fear AND respect Merryweather. And perhaps it would set up Merryweather as a possible antagonist. An antagonist that might be a lot more frightening this time around...
edit: If they operated within GTA 6, it would have to be what are called "grey-area contracts" or illegal operations. Which would essentially turn Merryweather into a bunch of criminals. And they'd be risking a lot. So if they did operate in Vice City. They'd have to REALLY be after something. Credit to Own-Papaya for pointing out they lost the ability to legally operate in the US.