What are the powers of Admiral Green Bull? I bet you have already thought about powers connected with nature, however, in this theory I will show you a power that can give us indications of your arrival in Wano!
I agree that Greenbull will have a mythical Zoan fruit, but I think it's a bit more on the nose as to what it would be.
He's a green bull.
The only bull that's green is a bullfrog.
And it just so happens that there is a mythical creature resembling a Bullfrog
The Jin Chan
(A picture of the Jin Chan)
The Jin Chan is usually depicted as a bullfrog with red eyes, flared nostrils and only one hind leg (for a total of three legs), sitting on a pile of traditional Chinese cash, with a coin in its mouth. On its back, it often displays seven diamond spots. According to Feng Shui beliefs, Jin Chan helps attract and protect wealth, and guards against bad luck. (Source: Wikipedia)
This could also directly give us a hint as to what his ability would be.
I think the ability to create and manipulate valuable objects like coins or gold could be possible, but it could also have an ability related to luck in general, like we see in Film Gold with Baccarat's Raki Raki (Luck-Luck) no Mi, but it instead only gives the user or an affected
person the inability to be unlucky instead of being extremely lucky.
It could also give Greenbull an ability similar to Marco, that he can turn into coins or gold like a Logia user.
But this is just my Headcanon, so take it with a grain of salt.