I'm not big on some of these huge UFA names who have either already won their cup and/or are looking to cash in massively. Plus, I don't see us making someone the new highest paid player on the team, which is what a player like Rantanen would become.
If we're trying to add another truly high level player, I'd rather do it via trade and shell out the assets for someone who has some term and a cap hit that doesn't exploit the increased ceiling, i.e a Thompson/Pettersson(Edit: just realized Pettersson is 11+, thought he was 9.5 for some reason) instead of a Rantanen or Marner. Sheer examples here, not saying I'd pursue.
I would check in on Jamie Benn to see if he'd take a short term deal at a decent rate, chase a cup type of thing.