Ok, just kidding. But a Venture Bros./Batman crossover would be awesome. Hank would obviously become the next Robin and die, Dean would have a massive crush on Barbara Gordon, and Brock would sleep with Catwoman and slit the Joker's throat. It could happen, right? The Cartoon Network and DC are corporate siblings. Anyway...
It's actually The Monarch Of Menace!
The Monarch of Menace, dressed as a king complete with crown, robe, and scepter, robbed a bank many years ago. Batman quickly dispatched the henchmen, but the Monarch of Menace took off on foot as his special shoes left a trail of glue with every step. Unprepared, Batman was defeated by the glue, electrified scepter, and poison gas from his robe, and the Monarch of Menace retired from crime to bask in the glory of being the one guy Batman never caught.
Now, the Monarch's son is fed up with his father getting all the glory, so he steals the Monarch suit and decides to make his own mark as a master criminal. Robin easily catches him, as he doesn't know how to use his father's suit, but the press goes crazy with the news that the Monarch of Menace has finally been brought to justice. Evil doers can't stand it when their villainous reputation is besmirched (reading all these old comics really makes me want to write like that), so the original Monarch of Menace comes out of retirement. Batman's ready for his tricks this time and easily dispatches the Monarch of Menace, and the caped crusader's crimefighting record is again unblemished.
Pros: He's extraordinarily unknown, having only appeared in the comics a few times, so even a lot of fans have no idea who he is. That means that announcing him as the villain would cause sales of the collection with his origin to skyrocket.
Cons: I doubt that very many copies of that collection are in print, so it'd cause eBay prices to skyrocket more than actual sales. And the Monarch of Menace is a gimmick villain with nothing to offer besides the king theme.
How I'd do it: I just wrote a synopsis for Venture Bros. fan fiction where they meet Batman, so I'm not sure I'm in any position to say how I'd write any story.
Casting Guesses: He hides behind his mask the entire time, so physically, he could literally be anyone. But The Monarch actually delivers the line "I herewith dub thee Sir Fool, Batman, for thinking you could ever capture the Monarch of Menace!" I can only imagine a guy like Eric Idle saying that. Or Mark Heap.
Verdict: As usual for Not In A Million Years Week, no chance.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.