My guess is that they'll kill off Charlie or put him in rehab (extended) and that Kutcher is the caretaker or beneficiary (multiple backstories here on how this came to be). Another option is Charlie is dead, Alan gets the house and takes on a boarder or is forced to (BIL for a girlfriend etc.)
Come to think of it, the writers are a bunch of lazy asses...they're probably waiting for us to come up with something neat.
It will be as lame as Sanford and Son, when they had Grady stand in for Fred, while Redd Foxx was in a contract dispute.