I'd like to see what he can do. It all depends on the material he's given. With Dafoe, it can go either way. He can easily go into Jared Leto territory, if the script and character are hollowed. However, in a Joker/ TDK type film, I could see him being great. In an 89 Batman movie, I could see him being good too, but Jack is hard to top when he's doing Jack.
I'm not a fan of his Green Goblin, which is what his Joker would most likely be like in a mainstream big budget Batman movie. Not Leto bad, but not great either. He's a bit too old now. He's 64, so he probably moves like a 64 year old man, and I think we all saw what that looked like in The Irishman, not good. He still has the face for it, if anything, he looks even more like Joker the older he gets. With him, less makeup would be the key for a great looking Joker, because too much makeup would hide his features. The Ledger, Nicholson, and Phoenix makeups wouldn't work. Ironically, I'd pick Leto's makeup, so that he still looks like Dafoe.
With him, I'd go for a Golden Age Joker in the 1940's, mainly because of his age. If he was young, I'd go for a Streets of Fire setting for his Joker, not 40's gangster suit.