Well, I found these concept art online of BWTB, and found two specific ones of certain intrest.
These two concept sketches resemble a certain homicidal clown. Not supriseing, as the decision to use lesser known villains came latter on in production.
Well, these reminded me of certain incarnations of the Joker.
The Joker from NBTAS and Frank Millar's Dark Knight Returns. The later one would fit in very well with the dark mood of the show.
It also reminded me of another Joker concept art I've seen online:This was drawn by singer and writer Gerard Way for a limited series in DC's Vertigo magazine that was never made known as "Batman: KIngdom Of The Mad".
It's also dark and would fit in well with the show. It even would work well with CGI!!
So, how would The Joker look in BWTB?
In my oppinion, It'd be something like this (beware of bad photoshops):So, I based this design mostly off of the B:DKR design, at least, color wise. So, in DKR, Joker just has really f*cking pale skin. Like, really f*cking pale. He might even be an albino (with the grey eyes and all). So I went down that routte. Joker's eyes are a very pale blue, light bluish grey, and his hair I'm somewhere between mulchy green and black. His suit: Green tie with polka dots (as per the concept art), purple or black button up shirt, and a black or white sportscoat or peacoat.
Also, the bloodstained scrubs for when he's locked up in supermax/miskatonic/arkham/whatever.
Ok, but how would he fit in with the show?
Well, seince the creators stated they didn't want to use him as a villain, make him more of a hannibal lecter type: He's locked up in prison, but he can still get sh*t done, and can also provide Batman with information when needed. Or, he could be Simon Stagg ofter falling into a vat of chemicals....NAH!
So, Joker was one of the first crimminals Batman encountered, towards the end of his first year as Batman. A man known as The Joker was predicting murders on the radio, and would suceed regardless of police attempts to stop him. Batman eventually was able to figure out Joker's plan to poison the Gotham resevoir, and was able to stop him. Joker was then sent to Supermax/Miskatonic/Arkham/whatever, and was locked in maxumum security.
Joker would probably act as an occational helper to Batman, giving him important info if needed, but also trying to get inside the batman's head, and drive him insane. Simmilar to Hannibal Lecter in Silence Of The Lambs.
He could also be secretly working with Anarky, or other villains, etc. And there would be a possibility of him escapeing, with the help of Humpty Dumpty, and it would be simmilar to the Joker plot in DKR.