I've been thinking about this for awhile, but maybe Opeth should find a designated second vocalist to do all the harsh vocals live, and maybe even in-studio. I'm not sure if Mikael would be interested, but it could potentially work out wonderfully. You would just have to find someone who can do old-Opeth growls justice.
My personal recommendation would be fellow Swedish vocalist Christian Älvestam (Solution .45, Miseration, ex-Scar Symmetry). He's a great vocal talent, clean and harsh. His low guttural growls rival Akerfeldts and sound amazing. It would be almost a perfect replacement, imo. and again, his Swedish brotherhood would be a good fit for the rest of the band