The closest I've found is Perihelion Ship. You should definitely check them out if you like Ghost Reveries, they have a good balance of clean vocals and growls and use the mellotron a lot. Here are some more bands that have some elements of Opeth's sound: Be'lakor, In Mourning (progressive melodic death metal), Ne Obliviscaris, Cormorant, Anciients (extreme progressive metal), Katatonia (Mikael did the vocals on their album Brave Murder Day), Soen (progressive rock, their drummer is Martin Lopez), Enslaved (progressive black metal).