Arceus was a great addition to the series because its lore helps people truly into understanding the world of Pokémon try to piece everything together and form connections, whether within the same generation or cross-generational. Pokémon is a series after all, so a lot of what previous gens established has and will be utilized in modern and future Pokémon games.
Arceus is a simple answer to everything with other Pokémon forming as more layers of complexity to serve as a more direct cause behind what goes on in Pokémon.
For example:
Arceus -> Plate -> Xerneas/Yveltal -> Ultimate Weapon -> Mega Evolution
Arceus -> Palkia/Dialga -> Many layers of dimensions (let's use Ultra Space as an example) -> Ultra Wormholes/Space-Time Rifts, Space-Time Distortions
Arceus -> Plate -> Kyogre -> the ocean
Here's what I believe personally based on a ton of research:
Arceus -> Plate -> Xerneas/Yveltal -> life force + a Pokémon's own unique characteristics -> generational phenomenon.
For example
Life force + the refraction of light from Necrozma's prism = Z-Moves
Life force + Eternatus = Dynamax
Life force + some fancy turtle boi = Terastallization.
I believe the powers of Xerneas and Yveltal are what perpetuate all phenomenon in the world of Pokémon. I also believe they derive from the plates of Arceus, which the Arceus movie refers to as life plates, as they give life to environmentals and to Arceus itself. They are fragments of Arceus, which the Japanese translation of the Meadow plate confirms is the origin of all Pokémon powers. Legends Plate in the English translation sorta backs this up by saying it's the essence of all creation.
This idea would also back up about Xerneas and Yveltal several months ago here
Copypasting a post I made somewhere a couple of days ago.
A lot of completely wack stuff coming from me that I've built up throughout this past month. Here goes!
I believe Reshiram and Zekrom were responsible for Entralink's creation, as it's part of Unova and the region was created by them.
In addition, the manga showed a direct connection between the Light/Dark Stones and Dream World, including a moment where White (Hilda) got dragged into Dream World by being absorbed into the Light Stone. As we know the Light Stone and Dark Stones are just Reshiram and Zekrom's body shifted into a...
I think Arceus was created as a means to help is understand the world of Pokémon, to help us understand the power that's inside. Gen 4 fumbled on the executuon of this Pokémon and ruined the way it was meant to be interpreted but they've been taking steps to fix it since.
I also don't think Arceus's case contradicts Mew because the series has been actively trying to say Arceus wasn't a Pokémon when Mew was created. Arceus existed, but Arceus is Pokémon's god. The Pokémon itself is just an avatar it uses to interact with the world of people and Pokémon. It can be created at any point and any time, infinitely. Mew is literally just a Pokémon and always way. Arceus was never
just a Pokémon, but first and foremost, rather something that defines the very essence of all creation. This is parallel to how Palkia encompasses all of space, Dialga encompasses all of time, and Giratina embodies all antimatter. The Pokémon themselves are their visual personifications of their concepts. The avatars.
Oh uh tl;dr Arceus is great. Ikik may sound biased from a dude named
RANSEI but hey I gave off my explanations!