I subscribe to the theory. I tossed a few ideas around about him having a Time fruit back in the day. I use to do Final Fantasy Connection theories, and in a lot of video games Time magic is usually associated with the color green, so that's where I kinda got the idea from.
Also I made the prediction that the three current Admirals might have a "Motion" theme going on.
Kizaru represents acceleration.
Fujitora represents deceleration. (The heavier the gravity, the slower you become)
And if Green Bull has a time fruit, he could stop motion completely. Although it's pretty broken, and I am not sure how you could contend with something like that, I am sure Oda has a way to, if that's the route he chooses to go with Green Bull.
Still hoping for a Plant Fruit though so Brave Warriors of the Sea Usopp gets his time to finally shine come EoS.