The more I think about Dynamax / Gigantamax, the more I like it. But I would make the following changes to it.
>All existing Megas are now Gigantamax forms. They get all the expected stat boosts / type changes / ability changes.
>If a Pokemon Dynamaxes, it gets a 10 point boost to all six base stats plus a boost to At and Sp Atk.
>Z-Moves are reworked for Dynamaxing / Gigantamaxing. A Dynamaxed / Gigantamaxed Pokemon has access to all the standard typed-based Z-Moves that match it's typing, in addition to all its normal moves. If a Pokemon chooses to use a Z-Move, then it returns to normal the next turn (the usual three-turn limit still applies). Also, there are now 36 standard Z-Moves, two for each type, one utilizing Atk and the other utilizing Sp Atk. The Base Power for these standard Z-Moves is universally 180.