So we have the setup to the conflict of the game. An unknown mage is doing BAD STUFF and you're all likely AGAINST THAT. This will involve investigation and likely confrontation, with the BBEG and it's minions. How that confrontation plays out will be up to you, but along the way, we may learn more about the broader mage world, too.
As part of the 'social contract' to the game, let me know if this works for you.
Before we get too far, we can change directions. For example, if you want more broader mage world interaction, we can bring that in. I arbitrarily made the assumption you would be most of not all of the mages in the city, but it is not too late to expand that community.