You seem to be totally overlooking one of the most obvious things.
Which is that every single Admiral power has been related to their associated color.
Aokiji = Blue = Ice
Kizaru = Yellow = Light
Akainu = Red = Magma
Fujitora = Purple = Gravity (it's been established that the Japanese associate gravity with purple)
Whatever power Green Bull has…will unquestionably be associated with green.
Well, I initially thought about powers associated with green like plant manipulation, disease, radioactivity, acid,.. but I wasn't convinced so I changed my approach and tried to think about phenomena related to light and gravity, or atleast to gravity. And I could only think of time manipulation considering gravity as space manipulation. I'm not really convinved either, maybe acid or radioactivity are better bets.
So overlooking that there isn't much of any connection that Light and Gravity create that…points to Time Travel (???)
Oh no no, nothing so powerful and troublesome, I was thinking more along the lines of stopping time for a few seconds, slow it down, speed it up, etc..
This is a very poor way to wave away the color association. Just because a power like that was taken doesn't at all make this irrelevant. It's a fundamental issue your theory HAS to address. If you can find some info about the Japanese associating "time" with green then congratulations.
I searched a while ago but found nothing significant, and even though I've seen some connection between gravity and the color purple in videogames, there isn't much I can find in the internet, so who knows.
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I would find him having a zoan to be really weird, all admirals' fruits are about some kind of natural phenomena so I think fujitora and ryokugyuu both being paramecia is much more likely. I could see it if we were talking about admirals from around sengoku's generation, kong for instance.