I'm not sure if I want to say there's something that will forever be off limits for me.
However I would make sure that if I ever pick up some shadier topic, I wouldn't downplay it in any means (like it wasn't that bad because they love each other now TEEHEE). Also like @joannekwan said about portraying racism, abuse etc. I don't like to see those things romanticized, which is what happens pretty often.
Also the pure evil vs pure good is something, like @noctuidae, I don't think I'll ever want to feature in my stories. Grayscale is something I like when it's about characters and their morals. I'm not saying it can't be done well, but it's not something I'm interested in making. At least for now.
I think most important is to be aware of things you put in to your comics and for what reason.