It's true, the "Only Couple" thing gives them a certain invulnerability, but, they can always bring Carol and Darryl together.I think after the Day Trip with Michonne and Morgan, and the Camp Woodbury Talk with the Guv'ner, Rick has put Crazytown behind him. If they were to kill Maggie, I think they would do it at a time when the reaction from Glenn could be held off until next Season (IE: Next to last episode of Season, which no time to soak it in until next season or last episode). But, again, it's certainly not what I want to see happen
I'm inclined to think they'll keep Maggie and Glenn alive because they're the only couple on the show.
Does anyone think we'll see the cannibals at some point in the future?