I kind of want Joker's suit to look like this. Something stylish and cleaner than Heath's, but going back to thlong tailcoat.
My top choices for Joker are Matt Smith
I agree with you on both of these. I see a great deal of Mr J in Smith's performance as Doctor Who; he plays the character as a kind of benevolent maniac. For The Joker, he just needs to drop the benevolence!
I haven't really seen enough of his acting to know about his range, but the way that he uses his voice and his movement to convey an alien who looks like a human is impressive. I sense some real potential there.
As for the Joker's dress; I think clean lines and crushed velvet would be the way to go next time. Ledger's Joker gave us a version of Bermejo's filthy vagrant, with a bit of a steam-punk vibe, but it is possible to develop the aesthetic in a different direction. I think the key aspects of The Joker's character that inform his dress are that he has cut loose any moral or conventional bounds, and that he is a narcissistic popinjay. He wears
whatever he wants, and whatever he
finds amusing. He seems himself as the
clown prince. Between those influences, it is very plausible for him to display garish luxury, like a rap artist with more taste and a liking for vintage. So a beautifully cut, fuschia three-piece suit; polished cuban heels; a walking cane etc is very apt. It also seems likely that he 'perfects' his chemical-ravaged looks, by dying his hair a deeper green and painting his lips a more vibrant red.
This stuff is, of course, inherently camp, but so is the character. So, indeed, are a lot of real-life psychos: think how much attention the Yakuza or the 1920s mob paid to their appearance.
The way to achieve a great Joker who is different to Heath's is, in my opinion, to play up the other half of the character: murderous, joyous, elegant evil.
Edit: And definitely a vintage shtick- I would love to see him use a personalised Tommy gun with a

painted on the drum.