Well, I loved the cameo, but I don't think we will be seeing more of Scarecrow in the third movie. The cameo gave us a sense of continuity, which is great, but that was it. Now he is in Arkham, and it doesn't have to be like the comics, where everyone is escaping all the time.
Besides, we know that too many characters on screen is terrible for the plot. And I am not convinced that this "villains teaming up" is any good. I mean, we've seen it before, since the sixties, and it would have to be very well constructed so it doesn't sound like a movie in which some crazy-dressed bad guys decide they want to kill Batman. So, ok, we're nerds and we'd like to see, say, the Riddler and Catwoman on screen, or even Scarecrow again, but please have a STORY for them! Have a REASON for them to be there, and not just to amuse batfans, or we may end up in Schumacher era again.