I don't think there was an obvious hint at the next movie - probably because during the making of The Dark Knight they realized that they would have to rethink things with Heath being gone.
However with Batman being
and Two-Face
then I think you can guess at least how the next movie will unfold in its beginning.
The Scarecrow was never really a strong villian in the comics - he certainly is a Batman nemesis, but not on the scale of the Joker or R'as. So him being reduced to a
in the second movie is not much of a stretch. Truth be told he spends most of his time in Arkham Asylum in the books.