5. Penguin
This is as iconic as they get. Penguin -or, Oswald Cobblepot- first appeared in Detective Comics #58 (1941), and is one of Batman's oldest enemies. The character has last been seen on screen in Tim Burton's Batman Returns (1992), where he was portrayed by Danny DeVito. He's also made appearances in video games, animated series', the works. In comics however, he's lately been shown as more of a facilitator of sorts. However, that doesn't mean he should be trifled with. Between The Joker, Two-Face, Black Mask, and himself, Penguin has been waging a war for control of the Gotham underworld for years. The difference between him and other villains though is in his profile. Where others may prefer to keep in warehouses, subway stations, or whatever abandoned building they can find... Penguin likes to be front and centre. It is this, his "lair", more so than the character himself that I believe needs to be a part of the series. Penguin has been a part of the Rogues Gallery for years, but even when he isn't involved in a story, his club manages to pop-up quite frequently. The Iceberg Lounge has been an establishment in Gotham City for many a year, and well before the time of The Batman. It plays host to members of Gotham's elite, and its underbelly alike. It is in this fashion that I think Penguin needs to play a part in the series. I would be very disappointed if he and his club didn't show up from time to time.
Ideal Casting: If he appears as I'd like him to, with some sort of a recurring character along the lines of Person of Interest's, Elias, I would want somebody prolific and menacing to play the part. I can't think of anybody I'd rather see a suave and sophisticated criminal than Danny Houston.