I don't know how big this is but there's this new theory that Donny is just a figment of Walter's imagination. Like he's an old army buddy that had died or something. It almost works. There's the "your phone is ringin', Dude" "thanks, Donny?" [exchange]. But that's the only acknowledgement that the Dude makes of Donny. If you watch those scenes, it's like Donny would come in, Walter gets so upset and it's like the Dude never hears it.
