The latest Hobby Japan book dedicated to Godzilla vs Destoroyah had a nice section on unused concepts. The book is fantastic and covers a lot of new material that isnt in the Shogakukan "Godzilla vs Destoroyah Ultimate Collection" or the "Godzilla Heisei Perfection" or "Godzilla Chronicles" books.
There is a lot about Godzilla vs Barbaroi, but since thats been covered pretty well here are a few that I was less familiar with it.
Heisei Era Oxygen Destroyer and carrying case - a few of the scenarios ended with Godzilla dying to the oxygen destroyer again
“Godzilla vs. Biomonster” - This pitch featured a “bio-monster” designed as a modern Frankenstein. Skin is a modified version of G cells.
“Godzilla vs. Deep Sea Life” - This concept featured a monster from the deep fighting Godzilla and apparently resulted in G Force using the Oxygen Destroyer to destroy both monsters