I may be reading into things too deeply but Lauren Cohan (Maggie) seemed the most reserved and somewhat not present, she continued diverting to "we can't speak much about the next season" whilst everybody else found something to say during Comic Con interviews and panels and she is usually a really bubbly person.
I could totally see this happening. It would come as a shock, and it would be Glenn's other half which could level out his comic book fate with them just switching roles.