Stephens need to be there, even if its centering the 4th line again. I thought he was good at this year's tournament when he was on the ice (which unfortunately wasn't that often). I think Senyshyn from SSM has a good chance next year as well, as a very speedy natural winger who can find the back of the net.
I can't see Strome back next year. He almost made Arizona out of camp this year, and would have improved upon his weaknesses (strength and skating) by the time camp hits next fall. Arizona is a low-pressure situation for him, and I imagine management there would like him to be part of their nucleus of young, talented forwards going forward. Crouse will certainly be NHL-ready by next fall in terms of his size and style of game, the only question will be whether he's playing enough of a role for the Panthers that they don't release him.
I'm not sure about Marner. The kid needs to hit the weight room and increase his strength significantly. It's possible with the right S&C coaching and some dedication. He's always going to be slight of build as compared to your typical NHLer, but if you look at a guy like Konecny who's roughly the same height as Marner, his relative strength is superior based on testing at the combine last summer. And it shows in his game too.