Of course you can't replace him , but you can always clone the guy , and raise him with the same influences as the original mikael . You would never know how it would turn out , would the music be better or worse?
Anyway , that would take a while to do but.........Steve wilson could pull of the vocals I think , and Dan Swano has written alot of opethiesh stuff on Crimson , and i'm sure he could do better and sound more like Opeth , analyze there every chord change and progression and get it down path , then see if he could do it .
And i'm sure Peter could write some Opeth stuff . I personally don't believe that Peter didn't have any input on the writting of everything after Mayh