A couple of weeks ago, there was a mechanics change to the interaction between Judgment/Hidden Power and Dynamaxing; these moves are now changed to reflect the intended type when used as Max Moves. In light of this, I think it makes sense to apply this reasoning consistently and change the type/category of other Max Moves based on the nuances of the base move where it's intuitive. Currently, Photon Geyser changes category based on the user's stats, but Max Mindstorm based off of it is always a special attack; this means that people using Necrozma-DM have to run Psychic Fangs, a weaker move, in order to get the most out of Dynamaxing. I think the category of Max Mindstorm should also be changed dynamically based on the user's stats.
