Imaginary Fest should do what keeps F/GO going and introduce some new characters and not just variants the canon ones in different outfits.
There’s as much variety in potential mages in Toaru as there are Servants with all the mythologies they borrow from and emulate.
We could also see things the series failed to explore so far such as magic cabals mentioned only in word, the existence of vampires back in Volume 2, those other less than 20 Saints we still have yet to see, etc.