I'll probably design more Megas for this supposed DLC at some point, but I've only really thought about the big focus of the DLC itself, Eternatus.

For the design, I leaned into the Lernaean Hydra inspiration for the normal Eternatus instead of the G-Max Eternatus' Hand design. So, Eternatus' head is split into three. As a trade off, he's lost his ability to float and has become similar to Zygardes 50% form. It's height has doubled to 40 metres, and it's ability changes from Pressure to Max Pulse. Max Pulse acts similarly to Dynamax since... well, he is theorised to be the reason Dynamaxing and Gigantamaxing exists. Every 3 turns, Mega Eternatus can use any Max Move out of the 4 moves he has. So, if he has Poison Tail, he can use Max Ooze. When you Mega Evolve Eternatus in Battle, it starts with the 3 turn cooldown, and if you don't use the Max Move, the cooldown resets. In Z-A, this would most likely occur after 3 moves are used instead of 3 turns.

Eternatus also makes sense in terms of the DLC stories. Main Game; Zygarde was the Main Mega; Hyperspace had Rayquaza, and Eternatus is a Poison / Dragon. Furthermore, Dynamax Cannon would be replaced with Eternal Cannon. For one, Dynamax Cannon will now deal extra damage to Steel Types, about 1.15. Eternal Cannon on the other hand acts similarly to Nihil Light, only dealing Not Very Effective Damage to Fairy types instead. There is a cooldown, just like Eternabeam.