Middle relievers often don't get much love from fans or the media alike, but Phil Maton was a really valuable bullpen piece for the Astros for 2.5 seasons and saw his contributions fly under the radar. He struck out better than a batter per inning pitched the last two years and is good for a 3-something ERA almost every season. He just does his job.
The problem with Maton is that because he doesn't throw hard, teams that prioritize analytics can penalize him a bit. Combine that with the fact that he is 30 years old and has some issues with walks, and it could be mildly difficult for Maton to find a team that will pay him what he is worth. The Astros could bring him back (and they should), but other teams like the Cardinals are interested in Maton's elite ability to get soft contact.
Zack Greinke continues to defy all logic 20 years into his future Hall of Fame career. Greinke pitched in Houston for the 2020 and 2021 seasons and while he wasn't his formerly dominant self, he did still manage a 4.12 ERA in 42 appearances. After leaving the Astros, he returned to the Royals where he started his career before becoming a free agent this offseason.
It sounds like if Greinke wants to come back and play another season, the Royals would happily take him. However, as of early November, he was still undecided if he was going to come back for another year or head off into retirement. Whatever happens, he has been a treasure for the game of baseball and it was fun that the Astros were a small part of his time in the league.