The Cosmic Crystal is a fourth dimensional construct that anchors the Spacetime Continuum and connects the End of Time with the Beginning of Time in the Eternal Loop. That was, of course, until the Timebreaker Wars, wherein gods from many different pantheons battled each other for control of the Crystal. The sheer ferocity of the conflict severely weakened the fabric of the Continuum over repeated cycles, until the entire timeline collapsed in on itself like a massive quantum black hole. The immense pressure of the implosion rendered the Cosmic Crystal unstable, and it exploded with such magnitude that reality itself was distorted and shattered.
The Spacetime Continuum began to heal itself, but since the Cosmic Crystal was now scattered into many different Quantum Shards, the Eternal Loop was forever broken. Instead of one unified timeline, there were now thousands, branching off into endless possibilities. And in some of these new timelines, the Quantum Shards bonded with hosts, marking them as Avatars of the Cosmic Crystal and instilling in them a deep-seeded desire to travel the Multiverse and unite with other Avatars to eventually reform the Crystal.
So, your characters are Avatars, and you can customize the timelines they are from however you want. Alternate pasts, alternate presents, alternate futures, whatever. Create to your heart's content. That goes for abilities, too. Consider this a power fantasy playground where we all play OP characters who are basically gods. Any time we have combat stuff, I'll just throw swarms of enemies at the characters and then we can just have fun. To give you an idea of how tough the various adversaries should be, we'll say pushovers and minions are C-Tier, somewhat capable opponents are B-Tier, mini-boss-type enemies are A-Tier, and formidable threats are S-Tier.
C-Tiers and B-Tiers require minimal effort to defeat, A-Tiers are a bit harder but shouldn't be too much of a struggle, and S-Tiers are quite powerful and dangerous if they're not handled strategically. That will give you an idea how to write out your battle scenes.
As for the initial plot, we'll say that each character has just discovered the Quantum Shard in their timeline and bonded with it, and they are then teleported to what is left of the floating island Celestia. This used to be a beautiful whitestone palace surrounding by a sprawling garden and sitting amidst a sea of multicolored cosmic energy, and the Holding Chamber inside the main tower housed the Cosmic Crystal. But now everything is shattered and rotting and covered in black vines, and as the Avatars gradually collect more Quantum Shards from across the Multiverse, Celestia will begin to heal and rebuild itself. As for how we get the Shards, we'll say that all the S-Tier enemies we face are fellow Avatars who became corrupted by their Shards and turned into Void Demons. So, we seek out Void Demons and their minions, defeat them, retrieve the Quantum Shards, and return them to the Holding Chamber to gradually reform the Cosmic Crystal.