Dynamax actually makes competitive Pokemon (2v2) more interesting as it can be used by any Pokemon and doesn't require an item. This means on-the-fly adaptation becomes key, battles are less predictable, players have more ways of staging a comeback, and it's just TENSE.
I say let's use some of the existing Mega designs as Gigantamax forms and use some others for proper evolutions provided they are nerfed (Kangaskhan, Pinsir, Audino, Houndoom, Mawile, Glalie). Basically you'd get the Mega designs (which is the main thing about Megas that people want back) and at least the attacking Z-moves are more or less included as well. For those who were really happy with a Dark-type Gyarados and Dragon-type Ampharos, there will always be regional variants. Everyone gets their wish.
I hate Gigantamax as a word though. How about... Megamax. 🤔