1- Imu has one of the Road Ponegliffs:
Shiryu: ''I bet he's with the Government. It wouldn't be surprising if they kept a stone or two for themselves... It'd be a surefire way of preventing any pirate from ever claiming the One Piece.''
Definitely make sense as Shiryu knows how the Government works, he gives a good explanation on what happened to the Road Ponegliff at Fishman Island.
It would be a retarded thing for Government to not take any precaution after Roger find the final island. Government stealing 1 one of the Road Ponegliffs means they need to defeat the Imu and Government to reach the final island.
2- Oda revealed Imu too soon (unlike Kaguya from Naruto) to be a disguised FV:
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Chapter 906
If Imu was suppose to be an opponent after finding OP, then revealing Imu as disguised FV this early makes no sense. Now its lost the surprise factor and the audience has no connection to Imu as a villain since Imu wasn't build up through the story.
Either build up a villain for sometime, or disguise him and make him show up when no one knew it existed (such as Kaguya from Naruto).
3- Imu and Saturn both hyped Teach:
It would make no sense Imu and Saturn hyping Teach but not take any role in the final battle vs them where Imu falls.
4- Rayleigh is already complaining that the secrets will be revealed before finding Laugh Tale:
''Don't be an ass and take it away from the youngsters''
Foreshadowing that the secrets can be learned while fighting against Imu.
If you look at the previous fights, Oda's style is showing the flashback during the middle of the battle not in previous island; Nico Robin Ohara flashback were in Enies Lobby not in Water 7, or Oden's flashback was in Wano not on Dressrosa or Zou. Oda will probably show the Imu flashbacks while fighting against him on Marijoa not on Laugh Tale.
5- Luffy will be hunted by literally everyone if he becomes the PK first, which will make Luffy's job impossible vs Imu:
''Thanks to that title (PK) , the whole world is after us.''
So the logical conclusion is that, Luffy and others should defeat the Government and Imu before someone becomes the PK.
Otherwise all the pirates who joined in OP race will attack PK, this will only make things easier for Imu and Government to win.
6- Oda said the series will be over when they find the One Piece:
''Aoyama: That's a secret, haha. I might decide to redraw the finale in the future… Hahaha.
Oda: I've always pictured what the final chapter of ‘ONE PIECE’ would be like, though from time to time, the situation surrounding it changes. It will still be over whenever the One Piece is found in the series, haha. Anyway, this next arc is the final one.
Aoyama: Seriously? Is it really going to end?
Oda: Yup, to me this is really the final arc.''