What if the Makaiju aliens arrived at the same time as the Black Moon Clan?
What if the Death Busters began their attack concurrently with the Dark Kingdom?
What if the Outers had succeeded in killing Hotaru before she awakened as either Mistress 9 or Sailor Saturn?
What if Chibi-Moon had not come back to the past after the Black Moon arc?
Edit: I forgot a big one.
What if the Guardians never recovered their memories after Classic?