Came in here to disagree with ideas:
-Mirai Nikki is done-deal. What else is there to even do with that kind of ending?
-Black Lagoon. We know all about it by now; who they are, what they do, what the protagonist wants to do and whether he's accomplished that. Another season entails more episodic content that likely won't amount to concluding a bigger picture (which doesn't exist). In other words, it probably won't go anywhere.
While I have my disagreements, I will admit:
-Deadman Wonderland set itself up for a sequel despite bombing its pacing. Lots of questions have been left unanswered and it is possible for it to redeem itself if it follows the manga minus the dumb lines the protagonist is given.
-Haruhi could certainly use another film that takes place some time after Disappearance, though I'm also content with leaving it as is.
-Like Haruhi, Madoka can follow up the events after Rebellion. A movie would be great, I'm sure 12 episodes may be more time than needed to get the point across. At the same time, I'm content with leaving it open and treating it as an end.
-Rosario+Vampire Capu2, only if it ditches the childish fanservice and stick with the manga as closely as possible.