It would be really cool to be able to play mid-set formats, even if it is not in a league. For example, FOTR or FOTR + MOM formats.
I know that would make the format menu a little unwieldy, so maybe a league is the perfect way to implement this.
Some of those formats are pretty cool. The early FOTR formats are like playing FOTR without Horn, Heir, or Keeper, among other cards. Totally different format, and not in a bad way.
Similarly, I'd love to play a 1-8 or 1-8 plus 10 format someday -- Movie Block without Reflections.