I think the whole point of OG was to show the army experience through MD, so it would have to jump the shark before they could refocus on another character.
I may be too influenced by ff but TG would need to find a subject to hang it on, be that a conflict/deployment or another challenge common to the army. Given that they need some goodwill and cooperation from the forces I doubt they would do anything too controversial like bullying or women in the frontline as it could be seen as too negative. If it were me I'd want to wait another year or so to give a chance for MD's career to move on, which would provide more scope for a story.
I can't help thinking TG should have written another series between the pilot and this one. This series would be a good conclusion to the story of Molly.