I think regarding the endings, there's usually justification from a story standpoint for either (Killing Trevor because Michael has a family, killing Michael because Trevor arguably holds less of the blame for all the problems) and such a thing will be present for Lucia and Jason. I'm making a big assumption based on little, but considering Lucia starts the story getting out of prison and doing community service (and what we here from her in the other trailer), I'm predicting she's the one who might escalate Jason's casual criminality (robbing liquor stores) into larger and more intricate elements of the crime world.
One of the gangsters in the trailer accuses Jason of being a fed so he's probably not ingrained into things as she is. Alternatively, since we see hints of a corrupt police force, maybe Lucia got some sort of early release due to cutting a deal like Michael, where her previous connections to the city's criminal world enable her to get in, do a job for them to earn trust, then provide info that brings them down — Jason being accused of a fed might be due to that, where a previous gang was toppled and the only new element on the scene is Jason. Regardless, like Red Dead Redemption 1, after she's done her job they want her dead because she knows too much (it's likely the people she cuts a deal with are doing it under the table, have their own criminal operation, or at least seize some of the ill-gotten gains for themselves).
So at the end it'll come down to Lucia shouldering the blame for Jason ending up in their current situation (I'm the reason he's here, it's my fault, I can't let him die because I love him, etcetera) or Jason sacrificing himself to enable her escape and give her a fresh start (This is the only way she'll be free, she can have a fresh start, I love her, etcetera). Similar to how killing either Michael or Trevor in GTA 5 wasn't 'happy' in tone, it'd be the same here since. Killing either will be enough for the cops to ignore the other, or the survivor knows where evidence of the 'deal' is that allows one of them to get off with immunity or something. It's also how both of them would be able to get off and survive in such a circumstance.
A fourth option I didn't consider until now is to make is truly Bonnie and Clyde-esque, where neither sacrifice themselves and they die together, which can also be kino. Maybe that'll be the actual 3rd option presented, where both characters surviving and riding off into the sunset will be the win condition for holding off hordes of enemies. or something. A similar example of this is in Spec Ops: The Line, where you only unlock the "Welcome to Dubai" ending if you manage to kill all the soldiers at the end.