okay, I'll give some of my headcannons, and I hope I get back to it one day either to laugh at it, or to say I was right:
1- Last 3 SH recruitment: Momo and both Pedro and Okiku will follow!
2- Robin will die after successfully giving back the knowledge of reading poneglyph to Kozoki Lineage (Momo)... but she will find out the truth of the world/void century/ achieving her dream before dying... basically Robin doing what her mothers last words are... carrying the message to future generation as her final moment of death
3- This was very recent... just few moments ago I came up with... The Frog with Usopp is Yamato... Kaido's son.. he's in his full zoan form... he'll have an ability to release oil and actually do fast healing to make Usopp lie come true
4- Sanji will activate his exoskeleton in the climax of his fight this war...
5- Zoro will get the Nidai after his Sandai breaks this arc... and the Nidai will be waaay too powerful, even stronger than Enma despite being in same rank, because Nidai will be cursed...
6- Zoro opening his eye is a boost for his observational skills... so it's not haki boost or curse boost... rather his observation and reading the movement of enemy will spike tremendously...
7- Kidd will drop the Giant sword on Kaido or Big Mom, it will be his final biggest attack during that arc...
8- The Gorosei will start a purge during their formal introduction in combat.. in process... they will eliminate SOME BIG NAMES...
9- Imu has the power Of "creation" Paramecia...the father of all DFs, and the most powerful of all types... it's God's ability basically... and naturally he'll have max haki level in all types... I can imagine those big statues in Marojoa who look like soldiers... they are his army... and Gorosei are his royal guards... power of creation is so hax he can create ANYTHING in his imagination... literally anything... meaning.. create magma, lightening, tremors, darkness, armies, meteor, ice.. etc! Basically Luffy encounters all hax DFs he faced before in his last fight combined Lol
10- Xebec is alive
11- Xebec was a good man/NOT evil... and big named pirates followed him for different reasons... Kaido respected his strength... Big Mom saw the goal of "peace" with him.. saw him like mother caramel who understood her... WB respected him (only reason I can see why he was his right hand man) and I never doubt WB morality in those matters..
12- Roger learnt the truth of what Xebec was Seeking and what he knew about the darkness of the world LATER...
13- Shanks is Xebec son, and Roger respected Xebec enough to take his son and raise him... same way WB/Garp took Roger son under their wing
14- Xebec was called a "demon" only because he was looking into the taboo of the world which angers CD/Gorosei/Imu-sama... same way Nico Robin is called "devil's child" by spreading that they are devil because they are searching the truth...
15- Garp will die at Wano war arc.
16- Sabo and Bonney will be sent to Wano by Kuma as his final act to help Revs... reason he chose wano is it is isolated from WG reach... Sabo will hear of citizens talk about war on Kaido, then participates