The main gimmick will in fact be somewhat similar to be mega evolution and ultra burst.
Its called hyper phasing and has to do with time travel and how pokemon can undergo different forms of transformations throughout time.

like how mewtwo could get 2 megas, most pokemon can get 2 hyper phases with the prefix scarlet or violet at the end, representing the past or the future.

gameplay-wise it can be compared to mega evolution because hyper phases get +100 in their stats, but not every hyper phase is limited to that. The boxart legendaries get a "drastic boost to their stats" for example. Its also comparable to ultra burst since there is just one universal item for pokemon to hyper phase called the hyper crystal. Not every pokemon can hyper phase, only some pokemon can, like mega evolution.

They can get different abilities and different types too like mega evolution. Its very similar to mega evolution, the designs are just more limited by the concept of past vs future

It will be activated in battle by selecting hyper phase and then selecting your move, only if your pokemon is holding the hyper shard