Melkay, if you haven't considered this, what if prior to Batman besting him, Nygma/Nashton was a proud man who had some kind of social disconection. He's intelligent, but has a hard time mingling with his colleagues. He's awkward in social situations. After Batman would evade a set up Nygma/Nashton concocted to capture him, Nygma/Nashton's colleagues would make a few lighthearted jokes about being beaten by the bat. Nygma/Nashton would take this the wrong way and end up creating his Riddler persona, and this would lead into Melkay's idea.
Hope you don't mind I made the addition there. Give me your thoughts

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