Because Batman and Robin.
As I said elsewhere, a Robin- rather, a Nightwing- COULD work in the Baleverse, but as a copycat vigilante who's good at it and is gunning for Bats at first, only to team up near the end unwillingly.
And Green Arrow'd be better to toss in in that role than Dick Grayson.
But yes, Movie 3- Riddler as FBI profiler, trying to trap 'The bat' for murder of Dent, etc. Does so by engaging his detective nature, starts slipping more into the facade as times goes on. He still leaves clues as a tip off to his true self, and what he's up to, and becomes more and more the riddler as they fail to work, until he's pulling crazy ass shit to trap the bat, but also because he wants to now.
That's really the best way a Riddler could work in the Baleverse, methinks.
Penguin- just a short, squat mob boss- who HAETS the name penguin- who is restructuring and rebuilding the mafia after the DKR fiasco. Cuts a deal with Riddler somehow.
Also might work is Bane- bane as he is supposed to be, super smart, incredibly strong, and more than a match for the Bat. Make him a bounty hunter gunning for our Dark Hero's head.
Not necessarily all three, but hell, might as well try for some serious villain restructuring, no?
Though Bane takes the least, of course.