Ugh, I'm so braindead, sorry for posting in the wrong spot before, lost track of my tabs.
My crazy theories based on these spoilers:
-Green Bull is a fake-out: he's coming to help and heal Wano. Green Bull is the Shimotsuki Daimyo that helped Yamato, who managed to escape and join the Marines in hopes of someday freeing Wano.
-Having refused to eat until the people of Wano could as well, Green Bull will use his Plant Logia to make Wano a lush paradise once more
-Aokiji, spying on Black Beard for Sword, let the WG/Marines know that Black Beard is about to move on Wano
-Green Bull is also a member of Sword, and comes to let Wano they are about to be attacked by the Marines/Black Beard