I personally want to see Joker's Favor and Beware the Creeper made into mainstream comics because while I know that Mainstream Joker is very different (though not by much) the 90's cartoon Joker but I can see Joker get terrified of someone crazier then himself. Also it would be nice to see Joker have some comedic moments outside of his psychotic antics. Besides my favorite moments will always be when the Creeper attacked Joker and he went to Batman screaming "The man's a lunatic!" and when Joker cried out for Batman to save him from a normal guy like Charlie.
Now I know some will Joker wouldn't be afraid of someone threatening him but the thing you have to remember is that in a way Charlie and Creeper attacked him psychologically to. Charlie and Creeper both attacked Joker's identity in a sense. Charlie pretty much threatened to destroy his dream of a final showdown with Batman and Creeper as Joker put it was stealing his "act" so it would make sense in that regard for Joker to be afraid of them in those particular instances.
I'd also like that episode where Two face put a hit on himself and Batman told him he was his own worse enemy as well as the one where he developed a third personality called the Judge that was just an awesome episode especially the end where TwoFace plead guilty.
Another character I'd like to see become canon is Babydoll she was so crazy and at the same time she was such a sad character because she was an adult trapped in a child's body and it ruined her life. She was that little girl who had the TV show on the Batman series then grew up (not realy) and turned into a crazy criminal for those that dont remember.
I also wouldnt mind seeing Ethan Bennet and Yin become mainstream characters either.