I had a conversation with Oda the other day. He explained it all to me.
Shanks is working with his dad and brother in the background. Shanks joins his family in the end...against Luffy. He's gonna put on glasses and swoop his hair and throw the strawhat to the side when Luffy returns it. It's gonna be very Aizen-esque.
Shanks' crew is also there and in complete disbelief after Shanks attacks them, incapacitating like 90% of them.
Through tears, Luffy pleads with Shanks - but Shanks will be undeterred. They will fight and fight and fight. In the end, after a nakama punch - Luffy changes Shanks' mind and SHanks + crew join the SHs armada. Luffy fights Shamrock, Shanks fights his dad - and Luffy's bounty goes to 2.6 trillion beri.
Luffy officially declares war on the Celestial dragons....and Imu.
End the final arc - Act 2. Then another time skip into the final chapters of OP. Oda wants to have it complete by 2032 (35 years) - he told me.