It is WAY too soon to introduce a character like Hush.
His whole thing is manipulation of the other villains to make life hell for Bruce/Batman.
The Riddler including the word "hush" asn't a hint, it was simply a fun reference since he was bring up a scandal involving the Waynes.
I would love to see The Court of Owls come into play though Mr. Freeze makes the most sense as the main villain.
Strongly agree about Joker, he should stay behind bars for the whole franchise, with zero interaction between Batman (deleted scene isn't part of the story).