Theory by user three (This is actually his username i'm not kidding) and I also did a little bit this theory
This is a theory I came up with after hearing a line of dialogue in the english dubbed version of the film Godzilla vs. Biollante. It is different from the theory that Space Godzilla is a creature born from Biollante in that it assumes that Space Godzilla IS in fact Biollante, and not a warped cell or two that went into a black hole.
Dr. Shiragami, upon seeing Biollante's final form proclaimed "Biollante...she's evolving...". Her appearance becomes more grotesque and yet more Godzilla like. We know following the final battle she ascends into space, and from what I gather is never mentioned or heard from again until Space Godzilla appears. Now based on the assumption that Biollante is constantly changing and the process of evolution is taking place, I believe it to be more than possible (in fact highly likely) that Biollante is the creature known as Space Godzilla.
There are some obvious nods to the former creature SG was, such as the tusks on its face. However, it's ability to change forms between what it's smallest molecular structure is (crystaline) is similar to Biollante's (spores), as well as the odd spot on SG's abdomen, which surprise surprise is also a red area where Biollante's nucleus would have been previously. So, what does TK think of my little hypothesis? Is it good, bad, ugly, similar enough to the other one featuring Biollante and SG to be irrelevant?