Commander Cody/ The Clones
While Boba Fett and Captain Rex are the two most prominent choices for Morrison to play, there are a number of other Clone still around the galaxy who he could reprise his role as.
One name that comes to mind is Commander Cody, one of the highest ranking members of the Republic Army during the Clone Wars, last seen in Revenge of the Sith trying to terminate Obi-Wan Kenobi during Order 66.
Cody, along with many other clones, would go on to remain with the army as it transitioned into the Empire, and likely went on to continue serving with the Stormtroopers, as many clones did.
As we saw in the Mandalorian, despite the collapse of the Empire following Return of the Jedi, parts of the Empire are still functioning, with several Moffs and Stormtroopers still active around the Galaxy.
If Mando is seeking out a former Jedi in the form of Ashoka to help him with the child, it would make sense for Moff Gideon to dispatch a commander with experience facing force users, along with a legion of troopers, possibly one consisting of more clones.
This one is more of a long shot than Morrison appearing as Boba Fett or Captain Rex, but could be an interesting route to go for season 2 of the Mandalorian, and the potential stand off of Boba Fett, Captain Rex and Commander Cody, all men with the same face but who each walk a different path, could make for some very compelling television.
Stream The Mandalorian on Disney+. Season 2 premieres in October.