Teach wanted to join the WG because he wanted to go to Marijoa, IMO, like Rocks did.
Hachinosu would be protected by Marines, and Teach could sneak into the Flower Room the way he did Impel Down.
However, the Blackbeard Pirates got extraordinarily lucky: since Catarina Devon touched Saturn in Egghead, she most likely can transform into him and use the Abyss to travel to Marijoa. A Mark is what is needed to travel the Abyss, and no doubt Saturn had one. Imu just told us the Holy Land is under attack: like his Dad, Rocks, Teach showed up in the Holy Land and lit it up.
Teach's plan to make Hachinosu a Kingdom was before he:
A: Got Pluton and Poseidon's location from Caribou
B: Got Catarina's ability to travel to Marijoa via Saturn's form
Hachinosu is dangerous, but, it's a horrible defensive position: no borders, tons of area to land ships, no walls to protect from a Buster Call.
Wano? Wano has everything Teach wants.
Garp's going to get put through the ringer, though, he's probably the most hated Marine in service right now. Either him or Akainu.
Garp's execution will be in Wano.
In a pot of boiling oil.
Holding Tama, Yamato, and the Gifters above his head on a plank of wood.

