Definitely not batista since he was in the middle of retiring at the end of season 7. I dont think a promotion is necessary for the story. Deb would be the obvious choice if it had to be someone. No way quinn should pass up deb, and matthews would be a demotion (He is also in the process of retiring)
I don't really think the position of Captain is still relevant for the storyline. It was only relevant before b/c Lt. LaGuerta had been promoted, and she was a returning character.
So I don't think anyone should move into that position.