Maroon is an unrestricted free agent after this season. Earlier in the season, it was a cinch he wasn't coming back next season. His play of late has improved, but will that be enough to sway the front office to keep him? The next month may determine that.
Fabbri, Sanford, Sundqvist, Barbashev and Blais are all restricted free agents. The Blues will retain all of them; Sundqvist has earned a decent raise, Barbashev a smaller one. (Nikita Soshnikov, currently in San Antonio, will also be an RFA. The Blues would no doubt try to keep him as a depth forward.)
I have to confess I'm not positive on who would have to clear waivers to go to the minors. Right now, it's Thomas (though he can't go because of his age), Blais and MacEachern. Kyrou and Kostin should be waiver exempt next season, so they could bounce up and down.
They probably need to open at least one spot, for Kyrou, and maybe another for Blais. Kostin's play will determine where he goes. That's where training camp will come in. Does one or more of these guys beat out Sanford or MacEachern? Going into camp this season, Barbashev and Sundqvist seemed to be headed for San Antonio. Instead, they've become valuable pieces and Jaskin was shipped out. That's what will likely happen next season. If there's someone who they don't think is going to make the team and they risk losing to waivers, they'll try to trade them.
Thomas and Kyrou are essentially untouchable.