Ghost Godzilla retained the appearance of earlier Godzilla incarnations, looking much like the ShodaiGoji suit. It would have been at least somewhat transparent, glowing with blue colors.
Its dorsal plates were also in the signature maple-leaf shape of the original's, and it's possible that they would glow when Ghost Godzilla unleashed his atomic breath.
Godzilla vs. Ghost Godzilla
Ghost Godzilla was going to fight the current Godzilla.
Godzilla vs. Ghost Godzilla was scrapped because Godzilla's previous two Heisei enemies, MechaGodzilla and SpaceGodzilla, were like him. The creator said that "having three movies in a row where Godzilla fights a monster that resembles him is just too much," so the project was dropped in favor of Godzilla vs. Destoroyah.