The problem with players like Zetterlund, Peterson, and Kaut is that they do nothing particularly well at the NHL level. They're all fine - no obvious weaknesses - but none of them have a real plus NHL skill that allows them to fill a top six role. This allows them to be good at the AHL level, but not so much in the NHL.
Zetterlund was nothing special in his first two years in the AHL (combined 15 G - 23 A in 80 GP), and then had 24 G - 28 A in 58 games last season, at the age of 22. Dylan Gambrell had 20 G - 25 A in 51 games at exactly the same age in his AHL rookie year. Now, that's obviously cherry-picking, but Gambrell has sort of the same problem as Zetterlund, in that he just doesn't do anything particularly well - he's a decent skater but nothing special, doesn't have much of a shot, isn't strong, doesn't win faceoffs, not much of a passer or offensively creative. They're really not that dissimilar in this sense - they don't have anything that can fit in a top six role, they're just kind of there, taking up space and sometimes picking up points. On a good team, players like this are in the bottom six if they're not on the bench. They're not quite replacement level, but they're not far from it.
We shouldn't expect anything really interesting from any of them.