The best way in my personal opinion still is riddler to be inspired by real life killer the zodiac if you ask me. This is it if he is going to be the main villain but if just a side villain then maybe how he is portrayed in the comics right now wearing the suit and tie being a crime boss and still be serious than comedic with wearing tights like the other versions in the past. The zodiac and riddler are alike except one is a little darker than the other but I want to see the ultimate dark psycho version of riddler in only this world batman is in because I am curious to see how works out.  
 
             

I think that t would work with the Zodiac Killer thing, but I don't think that all of his things should be murders.  He would, like the Killer, leave clues, but not always with his victims. He might leave a clue in a bank he robbed, or whatever.    I seem to be rambling.