why not?
I'm very interested in how Morgan went into that downward spiral and became better armed than Rick's group AND the Woodbury group combined. Between what happened to his son, betrayal by other survivors and how it affected him I'd say you could get at least two seasons out of that story.
The CDC one could start with the initial stages of the lockdown, stress between the military and the staff, the eventual loss of contact with the other governments. I think you could get one good season out of that story line.
Woodbury and the governor, I could see that one going for at least two to three seasons. Basically follow the storyline of the book.
They could do a story on another group but that would basically be splitting potential storyline between two shows.
They did a very good job on the battlestar Galactica subplot episode razor's edge about the Pegasus, they could have extended that for a few seasons if they wanted.