I was taking/reading one of those ScreenRant tests (yes, I was on Facebook), and I came across the notion that the Riddler actually has the highest IQ of any of Batman's villains. Which is fine, but the picture was some shot of multiple villains but the mastermind was in shadow.
Which made me wonder if they've ever done a bit where the mastermind is Eddie Nigma who isn't leaving clues...he's taken this up as part of his therapy for the clue-giving OCD. The therapist suggested he try small things, so he created a new identity--things that the Riddler wouldn't care about and that he doesn't care about so he has less of a need for the clue-giving. The identity of the new character probably contains a clue that it's the Riddler, but it's a small clue...like Mister Tree (because it's an Enigma wrapped in a puzzle wrapped in a mystery) or Kermit Bongo (alternate names for Conan and Drum, hence Conundrum).
This new identity is successful (Eddie's a smart guy), and when he slips up by leaving clues, the players might well assume that it's the Riddler being a copycat. Players get involved because, hey, it's a series of crimes. In fact, in a nod to one of the Dortmunder books, other criminals might want to get involved because the crimes are successful, so other foes of the PCs start showing up at the crimes. But the crimes only work because Nigma doesn't care about them, so he has less of a need to leave clues. Then the other criminals force him to take something he does care about, and suddenly it's much harder to control the OCD. (Heck, he might even want to be caught by this point because these guys are messing up his treatment! Except he can't leave the usual style of clues; instead he has to leave new subtle clues and hope the PCs are smart enough.)
If you have a Riddler-like villain (Conundrum from M&M, for example) that might be a running thing as part of the set of adventures, leading to the final confrontation, which is when the OCD is really reasserting itself, and the big crime is a combination of both the new style and the Riddler style. (If he makes it out of the session, then the real climax is saving the psychotherapist, who is at risk of being murdered because his treatment didn't work...even though he probably didn't tell Eddie to create a new persona.)
If you're doing something like riddles in an ICONS game, I'd give the willing players five minutes to solve the riddle without rolls, and then make it a pyramid test. The test wouldn't be modified in any way but one: Every roll doubles the amount of time spent. So if they get a massive success in one roll, that's an hour or five hours or a day, whatever you've decided the time increment is. (We'll assume five hours for demonstration.) If they take three rolls, then it's five-ten-twenty hours--basically a day. If they take five rolls, it's eighty hours--a bit more than three days. Once they start rolling, at any point they can spend a determination point for a success.
Here, have a half-assed Riddler...the Puzzler:
Prowess: 3 Coordination: 4 Strength: 3 Int: 7 Awareness: 3 Willpower: 5
Specialties: Puzzles (Expert) +2, Weapons (Guns)
Powers: Bashing (Question mark cane) 5, Gun or equipment as needed
Qualities: Three steps ahead of you; Has to leave clues; Has to be the smartest man in the room
If you wanted to give him a power, maybe he has Probability Control, but only to give his opponents bad luck when figuring out his clues. That makes the pyramid test more of a bartering system, where the players can figure it out faster if they spend points, but they get more points if they let him hinder them....which is kinda like the source material. Killer Gamemaster is somewhat like this, if I recall, though I haven't looked at his write up for a while.