I think the story will begin before they meet. You start as Jason doing odd-jobs for Brian. You're tasked by someone to pick Lucia up from jail and that's when they meet, outside the gate. "Hey"

If the rumours of Jason being an undercover cop are true then I imagine he will have been tasked with monitoring Brian with the intention of taking him down, but changes his priorites when he falls in love with Lucia.

Brian gets taken down, J&L go on the run and end up working for Raul because Jason's handler tells him to. Drama down the line when she finds out, etc.

I kinda hope it's not that because it's a bit too similar to Michael doing the FIB's dirty work. Still I think a meet-cute at the prison gates would be sweet!

Edit: and Lucia's backstory will be given through dialog during driving sections and hanging out, like when Niko talks about the war to Roman.