That is actually very dangerous, assuming that you indeed do not require an item to Dynamax.2. Wallbreakers
I have 4 attacks Hydreigon and Dynamax. Now Dark Pulse turns into Max Darkness which also lowers the Special Defence of whatever it hits, letting it wallbreak more easily. I have a hunch that Dynamaxing also removes the previous drawbacks of the moves, so Hydreigon can spam Max Dracoforce (hoping that is what they end up calling the max Dragon move) for 3 turns without worry.
Say, if Max Darkness off Dark Pulse had 140 BP, Specs Hydreigon could have a chance (albeit minimal, I admit) to 2HKO standard Chansey of all things with Max Darkness after rocks thanks to the Special Defense drop, and something that resists the move, like Clefable, would need very heavy investment to avoid the chance.
Who needs Z-Moves?
There's Ditto if Imposter works the same way as with Megas.That's a good question. Dunno if moves would work, but Red Card might? That'd be interesting if the meta meant you usually ran one mon with Red Card to stop set-up Dynamax sweepers or something.