I always see in fan fiction where the riddler is under the effects of fear toxin he always sees his dad. Which to me is kinda overplayed and bland. I don’t think the riddler is scared of his dad, sure he hates him but he isn’t scared of him. 

Edward is insanely selfish so I don’t even think he would really see anything, his world relies entirely on himself. Of course he has childhood trauma but it is very unlikely he would be scared of it. He is more likely angry about it. 

I instead feel like his fears rely more on not being seen. He wants the attention he lacked from his childhood. He is constantly chasing a rush of adrenaline and the thrill of a game. Under the effects of fear toxin it would likely be a nightmare of not having anyone able to see him, akin to a ghost. Or quite literally nothing. Being trapped in a black void, no games no puzzles to be had. Nothing to stimulate his ever racing mind, just trapped with nobody but himself and his thoughts.