I’m not sure if anyone else thought of this, but something hit me today. Riddler used Renewal as a riddle that no one seemed to catch on to. We know that when Riddler announced his presence to Gotham that he said he would kill until “the day of judgment”. Though that was only part of his warning. In his journal, Riddler says:
My life has been a cruel riddle I could not solve, suffocating my mind, no escape. But then today, I saw it, a single word sitting on this ledger sitting on the desk beside me -- "Renewal"
The flood was not only his retribution, but to “renew” Gotham. To hit the reset button on the system. Unmasking the corruption was not enough. Corruption was everywhere. Gotham was a cesspool. So, Riddler played god to purge the city and drown the rats. The Real Change. Renewal was the broken promise sold to him, so renewal would be the promise that he would deliver.