I think that with the riddler, his threat is in his concept, rather than with the Joker, who's threat is very much the man himself. The Riddler is by denotation an enigma, his modus operanti isn't about acting as a man, physically, it's about acting as a force of intelligence, as an idea.
So would you like to see the Riddler portrayed in the film as an idea ('more than just a man!') i.e. his physical self isn't overtly evident. We don't really need to see the Riddler for 75% of the film. That way Batman truly is fighting a riddle, rather than a variation of the Joker.
Also, do you want traditional mind-puzzle riddles? Like, "Forward I am heavy, backward I am not, What am I?" [INSERT BETTER ONE HERE] Because I'm personally worried that it will reduce the threat of the crimes by introducing a 'childish' gimmick - remember, the riddles - if they are in the film - have to seem very clever, but able to be grasped by the child audience when a solution is found. I'm worried that it'll be a bit like, "Oh crap, a criminal has planted a bomb and if I don't work out what this enigmatic clue is, we're all done for ----- Freeze whilst they solve the clue ----- pphhhew, good job we were witty enough to work out that NOT backwards is TON!"
Because that equates to Harry Potter.