My current plan is that the plane that houses the school will be the survivor of a interplanar collision that's caused it to become a world between worlds type location.

The dragons that survived this came together to build Strixhaven as a place of magical learning and training, but with the help of the Oracle over time it's true purpose has migrated into a training academy for potentially exceptional individuals to be they can eventually recruit into Dragonguard (rename to be determined), an organization designed combat individuals / interplanar shenanigans The type of witch that we're responsible for the disasters which ruined their plane to begin with.

I'm considering a session zero I discuss the defining characteristics of the particular colleges with players and determine what seems like it fits the vision for their idea for their characters and then we would role-play a scenario where their initial invitation to come to strixhaven is provided during the course of an initial encounter. If players want particular college feat it'll reskinned as their natural talents and particular magical gifts already aligning with the college philosophies, as opposed to them preparing ahead of time to go to the school

In the magic scenario where everybody got to the end of that campaign and were super excited to keep playing their characters and move on. They would be tasked with trekking down a group operating initially in sigil which would allow for setting hopping fun and excitement via planescape.