I think the most ideal realistic trade is Rakell, Smith, Pickering and a lottery protected 1st for Marner, which doesn't seem that dissimilar to what the Kessel trade was back in the day. I think the Penguins would need to add another prospect, but not a significant one. They should be trying to keep Pettersson if possible in that deal.
The Kessel deal was basically Kapanen, Harrington and a 1st for Kessel, and I can't imagine that Marner brings back much more (if at all) than that. Pickering, a prospect like Ilyin and a 1st for Marner seems to be decently aligned with that in value.