We know that the impending free agents are Charlie Morton, Dallas Keuchel, Marwin Gonzalez, Evan Gattis, Tony Sipp, Martin Maldonado, and Brian McCann. That is about 28% of the Astros playoff roster in 2018. Looking at that list, I would think that Gonzalez and Morton could be the primary targets for retention. It will cost a pretty penny though.
Also, Lance McCullers‘ status is up in the air at the moment as he could be facing some type of surgery. While the rumor is that he is facing Tommy John surgery, they are keeping a lid on it for now.