Honestly, I am interested to see where he goes form here. He took a 4-year vacation after the Oscar, and even now he's got no future movies listed. Scorsese has been talking about making some Rome and Caesar movies and/or shows for years now, with DiCaprio included, but I don't see them happening.
As for Joker, I think he'd make for a nice Period Piece 1920s Gangster Joker. He's got that Prohibition Era look to him, you know? Ledger sold the whole Anarchist thing, Phoenix's Inceloker was fine, but Leo has that air of Americana Class (yeah yeah, I know, that's more of a 50s thing; you get my point). Dapper Gangster Joker is what fits him. Maybe it's Gatsby influencing me, dunno.