This whole subject would make a great "alternative time line" series!
One thing that definitely seems to be getting overlooked here is the fact that Godzilla was
very much alive at the end of
G' vs Biollante, with
no indication that the Anti Nuclear Bacteria was going to have any signifcant effect on him.
That being the case, before getting to my own time line speculation let me offer this item for consideration [or debate] first: If the Futurians
hadn't tried to remove Godzilla from history than that would mean the only Godzilla of the Heisei series would be the one which appeared in 1985. The even more powerful Godzilla would never be [re]created in 1991, and since the events of
G' vs Mechagodzilla wouldn't happen Rodan wouldn't give his life energy to Godzilla - making him even stronger yet. Would this change in events have any significant effects on the Heisei series history? Do we all remember what we learned from the
Back to the Future trilogy about stuff like this? :teach:
Ok, even if the Futurians
hadn't arrived in 1991 one significant comment regarding the future they came from was made in
G' vs King Ghidorah that's still important to mention. After Emmi had defected from the Futurians and was explaining the real reason for their mission she says "Actually, Godzilla hasn't been seen since..." before being abruptly cut off. And although Emmi never finishes her story she definitely conveys the feeling that Godzilla had ceased to be a threat to Japan at some point. [Especially since she explains that Japan grows to become the most powerful nation on the planet.]
Since Godzilla obviously survives being injected with the ANB in
G' vs Biollante, than many of the events of the Heisei series time line would still occur, but with some potential differences.
The events of
G' vs Queen Mothra would generally unfold as before. The meteor which awakened both Godzilla and Battra would still hit the earth, and Mothra would still hatch and confront them. But since it was the '91 Godzilla who ultimately killed Battra, would the final battle neccessarily play out the same way against Godzilla '85? If not, than it's possible Battra might still live to fulfill his purpose and fly off into space to intercept another approaching meteor before it arrived in 1999, leaving Mothra to remain on Earth.
The events set in motion with
G' vs Mechagodzilla would never occur. Mechagodzilla wouldn't be created since the technology was based on information gained from Mecha King Ghidorah. [There would doubtlessly be a gazillion "Super X's" tho'!.

] Rodan would also never merge his life essence with Godzilla '91's resulting in his becoming even more powerful and having that super fire blast.
Rodan would be alive and well on his island but "Baby" might not even exist [an idea that many of us fans can only wish for :calvin: ] since it remains unclear whether or not his egg actually belonged to Godzilla '91. And if "Baby" didn't exist events which would draw Rodan away from his territory might never occur. And should Rodan set his sights on Japan he would, of course, end up fighting the military and Godzilla [Don'tcha love it?

] which might possibly put a whole
new sequence of events into motion.
Space Godzilla would still be created and arrive on Earth, thanks to the G cells being carried off into space as before. But since the G cells would be those of Godzilla '85 wouldn't it be possible that Space Godzilla might not evolve the same way? Mogera would not be there to help Godzilla '85 defeat Space Godzilla for the same reason that Mechagodzilla wouldn't exist, and Godzilla '85 could possibly lose the battle and be killed. If that happened than four kaiju would remain on Earth: Space Godzilla - who wants to conquer the world, Mothra - who would doubtlessly fight for Earth's survival, Rodan, and soon Destroyah - who would probably just view the other three as enimies to be exterminated.
If Godzilla '85 still destroyed Space Godzilla then Godzilla '85 would instead be one of Earth's four kaiju.
The power within Godzilla '85 might never reach the critical point and cause his final meltdown as it did to Godzilla '91 in
G' vs Destroyah. Since it was the Baas Island disaster which was the catalyst for causing Godzilla '91's unstable condition, Godzilla '85 might not be anywhere near the island when it disintegrates since, without "Baby", it wouldn't have become his territory.
The Destroyah creatures would still evolve as before since the Oxygen Destroyer was responsible for bringing them into existance.
Since Godzilla '85 may not suffer the meltdown which killed Godzilla '91 it would definitely be a fight to the death against Destroyah. If Godzilla '85 did manage to be the victor, which might not be the ultimate outcome considering how much damage Destroyah was able to do to the
more powerful Godzilla '91, then he would conceivably just continue to exist until such a time as, one way or another, he was no longer a threat to Japan -
just like Emmi had begun to explain in
G vs King Ghidorah:!:
If Destroyah instead killed Godzilla '85 then the "battle for Earth" between Mothra, Space Godzilla, Destroyah, and Rodan would probably be on, and what a potentially cool battle it could be! I think that it would easily come down to Space Godzilla vs Destroyah. I mean, what chance would an oversized moth and pteranodon really have against those two? And there you have it! My head hurts.... :goofy: