Honestly, if Phoenix is out-of-this-world amazing part of me still thinks they should just bring him in. There's always a chance that the entire Joker movie ends up being a lie, since it looks like it will be told from Joker's POV and Joker is probably THE most unreliable narrator you could possibly have. So it could all just be "one possible version" of events, which is what an Elseworlds tale is anyway.
Yeah, there'd be an age gap but I don't think it's the end of the world. I'm just saying, if Phoenix's Joker is super popular and Pattinson's Batman is super popular, they'd be crazy not to entertain the possibility of having them meet on screen.