He's still Luffy's rival. He has stated himself he intends on becoming Pirate King, so naturally that puts him at odds with Luffy. They only allied to take down Kaido, and indirectly in the process also target Doflamingo. In the grand scheme of things, Law is thinning out the competition at the top, but he knew he couldn't do it alone. Which is why he got Luffy involved since out of all the Worst Gen Pirates, he's the easiest to handle (what he initially thought) and in the process ended up being sucked into the Straw Hat pace. Law still has an agenda, so I doubt he'd join. And as I said before, he has a crew that he's made since childhood and he isn't the type to follow under anyone.
Don't see why Marco would join Luffy. He's a strong ally, like Law, but I don't see the nakama potential for him to follow under Luffy. If anything, he might actually take up Shanks' proposal to join him since one of the first thing he does when he arrives on Whitebeard's ship is to ask him to join him.