There is no tension in the 3rd act because other than a totally random line, there is zero mention that Riddler is building to some bigger act of terror. And 100%, Selena's storyline is absolutely forced in.
1. Riddler murders/scheme don't feel important because we are basically told to hate the people he is killing. Of course murder is terrible, but why is it so important to catch/stop the Riddler? This felt so underdeveloped. The "Riddler is the Zodiac" stuff does not really gel because we never see or feel society impact from him other a few glimpses of people outside of the funeral.
2. The "twist" that will "shake Gotham to its core" and "disrupt the entire city to its core" comes in seriously half baked. I get it overall - the city leaders lied to everyone about the Maroni crime bust success when they really just took it over for their own personal enrichment and used a noble Thomas Wayne endeavor to cover it up - but we feel virtually none of the side effects. We feel zero societal breakdown. The detective stuff and maze of personal dynamics is interesting, but frankly, the film would have benefitted from slightly less detective work and "Your dad was bad/no he was good/he was actually mixed/falcone is the real bad guy" and more of "society is breaking down with each reveal" type of stuff.
3.. The "flood" was almost entirely shown in the trailers and isn't shown with nearly the impact that it would have had in real life. The entire ordeal feels rushed. Riddler only mentioned his "day of judgement" once before revealing the big thing to Bruce. They absolutely should have made that a larger part of his plan throughout the film to add a greater sense of urgency to stopping him.
Story really, really would benefited from being broken up into two separate, 130-140 minute films. Better establish the Maroni as Pablo Escobar type of character and make it a truly massive deal that he is apprehended. Better explain the Gotham renewal backstory. Give Catwoman and Batman more time to flesh out their paths. Flesh out Bella Real. Hype up the Riddler's "day of judgement" stuff. Would take a very solid, good movie and turn it into 2 likely classic films.