Alan though is a "hub" or "nexus" character - it seems that all the characters' plot lines run through or have the capacity to run through him. And this works, because it avoids that lame literary device of 'Isn't it a coincidence that so-and-so just so happens to have this relationship with such-and-such?' I think the writers would want to best leave Alan alone and bring in another character. Could even be a housekeeper for the Eppses, or a janitor in the FBI building, or a never-before-seen relative of one of the other characters. There are all kinds of possibilities.