I can't entirely disagree with that idea. I can't see them putting anyone such as Ronin Naruto, Rinnesasuke, Susano'o or Kimono Shisui, and Release Madara (Still not old enough, otherwise he'd be more probable).

However, I don't think that 5 Kage Madara's as low of a chance as you may think. Fireworks ninja tend to not be duplicates, which also eliminates normal/Koto Amatsukami Shisui. 5 Kage Madara's been out for a long time now, and his relevance to the modern day meta's not too high, from what I've noticed. His usage is relatively low, and while he's good, he's not nearly as handy as he once was. I can see them putting out 5 Kage Madara, due to the fact that they are both not too relevant, and old enough to appear. We've gotten crazier ninjas, I'd say. This one wouldn't be too much of a surprise.