With the Michigan governor, it was cool that they took from real life (Dearborn having a huge Muslim population). My immediate thought was that it was also a jab at the actual Governor of Michigan, who has proven himself to be a major slimeball with regards to the Flint water crisis.
For the survivor at the end, the film degree part of my brain says it has to be somebody who would supercede President Kirkman in the chain of command because that would cause the most drama/conflict, although if that person is healthy, they'd be able to take over and the show would be finished
. Chances are, if it's really someone like that, then they're somehow incapacitated to the point where they cannot fulfill the duties of President.
It'd be more likely to be a mid-level Senator, and the conflict can still remain of elected official vs appointed for who is more qualified to be the President, although since from the previews it seems like they're gonna try to use that angle to a degree with the Republican designated survivor (the senator from somewhere), maybe not that.
Hell, it's probably somebody completely inconsequential to the government, somebody's aide or adult child or something because that's how most cliffhangers work.