Hot Toys is the only company to pony up the cash for Nicholson's likeness. In fact, according to them, they had to meet up with him for his approval. They mentioned in their engrish how it wasn't a small feat and then bragged about how they were the only ones that obtained it.
NECA said it was too much for them when they were asked about Nicholson, so that's obviously where Mattel is at as well. Look at how cheap and profit oriented Mattel is with their other lines like Ghostbusters, BTTF and DC with their lame "you need to support our products with preordering even though we're showing you drawings printed out on cardboard", they'd never be able to get Nicholson, let alone set up a meeting with him.
So don't count on anyone but Hot Toys producing Joker figures with Nicholson's likeness. His likeness for collectibles is practically theirs, they obviously paid a good bit for it which is why they're constantly hinting at more versions after each one is revealed and advertised (Regular and Mime so far).