I'm not yet sure how he'd react to Lisa, though I'm more than happy to throw them together and see what happens!
Marcus might stay clear of Harry simply because of his reputation and Marcus' own history, since he's not very vocal on exactly why he joined the Death Eaters beyond fearing for his life if he didn't. Still, it might be worthwhile to see what happens, if you're up for it.
As for Pansy, I think they'd get along just fine, and if they've been friends for a while she's more than welcome to have figured him out, though he might not have admitted to it. I think it's be hysterical to see her try and set him up, though he would probably be at least somewhat resistant.