Every single EX boss at once would probably break the instance lol. But now that you mention it, it might be interesting to have two or three Primals at once, using mechanics that work together. A battle with all three members of the Warring Triad, for instance, would be interesting if they actually decided to work together instead of fight each other. Or imagine how epic Susano and Ravana teaming up would be. I think there's potential to make those kinds of fights fun while challenging.