Me personally, I'd have liked to have encountered Lock-Up. For those who don't know who he is, Lock-Up is a lesser known Batman foe who like Harley Quinn, was introduced in the animated series.
His real name is Lyle Bolton, and he was the head of security in Arkham Asylum. Durring his time as head of security, Arkham's escape rate dropped more then 51%. But like all things that seem too good to be true in Gotham, there was darkness behind this. Bolton used his position to abuse the inmates, both physically, and mentally. He denied them food exercise, and entertainment for even the tiniest infraction, electrified their cell bars and walls, chained them to the floors of their cells, beat them, tortured both physically and mentally for either info or sheer pleasure, and I would not be surprised if he even had sessions where he wanted to "have a party" with inmates. This gave Bolton the rare honor of being feared and hated by EVERY member of Batman's rogues gallery. Even Scarecrow feared him.
Naturally, when his cruelty was discovered, he was fired from Arkham. Bolton however was furious, saying that his methods while harsh and cruel, were necessary to keep Gotham safe. Bolton then decided that the people who run Gotham were just as guilty as the Arkham Inmates and Blackgate Prisoners, and became Lock-Up. His goal is to lock up anyone he deems a threat to Gotham City, which can include Batman villains, Gotham's elite, anyone he thinks is too soft, or sometimes anyone he doesn't like. Because of his brutal, knight templar methods, he has ironically become a member of Batman's rogues gallery.
As for how he would fit in an Arkham Game, if Arkham Asylum had DLC stories like the other games, I could see Lock-Up as the main villain of an Arkham Asylum prequel DLC. The plot would be that like Joker, he launches his own takeover of Arkham. But unlike Joker, he doesn't take over the Asylum to steal something. He takes over the Asylum to keep imprison the inmates and staff there FOREVER. So Batman needs to head to the Asylum to rescue the staff and inmates.
While there, you learn that some of the guards have betrayed Sharp and serve Lockup, and are allowed to do whatever they want to the rest of the staff and inmates, as long as they stay in the Asylum. Fight your way through rogue Arkham guards, rescue Arkham Staff and Batman foes alike, and learn that Lock-Up has much bigger plans for the Asylum and Gotham, and plans to turn all of Gotham into a supermax. One of the more unusual aspects of a DLC like this would be that you'd work alongside Batman's foes more then against them, even Joker, Penquin, Scarecrow, Killer Croc, etc. The story ends with the Asylum going back to normal, but Sharp getting some ideas from Lock-Up, hence all the new security in Arkham Sylum,
What villain would you have liked to see?