I still say Terrible Trio as an opening act, setting up something with... Killer Croc
They could set something up as "Similar to the Joker," but noting that something just... seems off. Maybe use the fact that it seems too choreographed? The Joker pulls things out of his ass and it happens to work. The Riddler plans everything to precise detail, even going so far as to taunt the police with Riddles...
And I just summed up the Riddler's MO...