Considering that the Seleucid Empire and the Ptolemaic Empire both arose out of the Alexandrian conquests I don't think it makes much sense to just have them spontaneously arise into existence. Now perhaps if Macedon conquers Persia and Egypt and then collapses or liberates them as colonies...that would make sense.
I see you'r point, but because Alex's empire lasted for a so short while Macedon will probably never concuer all these areas in time. Also something like that would probably need ALOT of coding. That's why leader change would be the best option, beacuse it is already in in the rhye's 'n' fall engine.