Scarecrow will return.
Dr. Crane is still out there somewhere, and the gang will have to face him again, with Batman coming to help this time. Scarecrow had ulterior motives for attending the parade.
Crane insists that he only wanted to watch the Halloween parade. But then, why would he cause chaos in the first place and boast about plunging the city into terror? It's possible he was merely lying through his teeth and just showed up to spread fear For the Evulz.