I've seen a couple of evil!Minerva stories (she turns out to be in league with Tom Riddle, etc.), but it would be interesting to make her an unlikable "good" person. Last year, there was an hp_beholder fic that made her into rather a harridan, as I recall, and I've seen some others where the emphasis is on her as a rigid, humorless disciplinarian making students' lives miserable. But she's not the main focus of any of these.