The original Tally Man grew up poor. His father owed money to a loan shark and they struggled to pay their debts. When his father died, matters only got worse. When a collector threatened his mother, young Tally Man killed him, spending most of the rest of his youth in juvie. When he got out, the rest of his family was all dead and he lost it. He apparently had a talent for shooting, and ended up working for the mob, killing people who couldn't pay their debts. He was completely nuts though, becoming downright gleeful while killing his victims.
The second Tally Man appeared during the Face the Face storyline (spoilers to follow). Working for the Great White Shark (who I just realized I haven't covered here), he kills Orca, The Ventriloquist, KGBeast, and Magpie, framing Two-Face for the crimes. He was stopped pretty quickly after his first appearance, and I don't think he's shown up since.
Pros: Essentialy just an mob hitman, either Tally Man fits with the presence of organized crime through each of the first two movies.
Cons: I doubt the mob would go to someone as obviously crazy as the first Tally Man after their experience with the Joker. And the second is a minor, minor character.
How I'd do it: I'd go with the second Tally Man. Just have him be a hired gun, nothing more to it.
Casting Guesses: The second Tally Man kinda looks like Mos Def, but bigger. I might go with Common, just because he's impressed me in the few movies I've seen him in.
Verdict: Almost no chance. There are so many other assassins to choose from. The only possibility is for a tiny cameo, in a situation where the better known assassins aren't available (because DC/WB is saving them for non-Batman movies).
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.