Beast is geared as a high-risk, high-reward offensive type, as several of the types it's strong against are also strong against it.

Raikou, Entei, and Suicine would be updated to Electric/Beast, Fire/Beast, and Water/Beast respectively, and the Rage attack changed to Beast type. Arceus' plate for the Beast type would be the Savage Plate.

I also thought about the Cosmic type, but it's apparently a very common fan-made type.

Here's my attempt at making a Pokemon more broken than Mega Rayquaza, but not childishly unimaginative like "It has an Ability that makes it immune to all damage!"

Rather than Mega Evolving with a Mega Stone and the Mega Evolve button, Arceus Mega Evolves by using Judgement while holding either the Golden Plate or the Silver Plate. When it Mega Evolves, Arceus changes to the Divine type, a new type exclusive to Arceus which hits all the other types neutral and is hit by all the other types neutral. Golden Mega Arceus is more attack oriented, while Silver Mega Arceus is defensive. Additionally, its Ability changes to Revocation, which disables the opponent's ability (except for another Mega Arceus' Revocation) and neutralizes any buffs (nerfs still take effect). The displayed message will be either "Arceus revoked [Opponent]'s [Ability]!" (Ex: "Arceus revoked Rayquaza's Delta Stream!"), or "Arceus revoked [Opponent]'s augments!" if also neutralizing buffs.

Additionally, rather than boosting the power of Divine-type attacks, the Golden Plate has the same effect as a Life Orb, while the Silver Plate works like Leftovers.

A while back on Twitter some folks were talking about how future Pokemon games should make it so an item cuts down trees instead of the Cut move, because Cut is useless in battle. Another answer could be to buff Cut.

Just making it a Steel-type attack instead of Normal would help. Or maybe even a Bug-type attack if you don't want to introduce a Steel attack that early in the game. And maybe make it a little stronger.

And just for giggles, Monstreon would be the Beast-type Eeveelution.

X and Y introduced Freeze-Dry, an Ice-type attack that is super effective against Water which normally resists Ice. Here's some ideas for more of these kinds of attacks.