
I think currently the war started between Government forces vs. SH Pirates / Revolutionary army since:
- Sabo learned about Imu, and become the Flame Emperor
- Vegapunk leaving Goverment
- Kizaru and the marines finally made his move towards SH Pirates
- Luffy showed that he wants to attack Marijoa due to the recent events
So its highly possible Oda will take out the Government war before the Blackbeard pirates battle.
While Shanks:
- Negged Admiral Greenbull
- Previously said he will go after One Piece
- Previously negged Eustass Kid, and Kid wanted revenge
Who will defeat Shanks, and when he will be defeated its really hard to guess.
Because I don't think Blackbeard will defeat Shanks, that would be a lazy writing. BB already scarred Shanks before, so there is no point to hype BB over Shanks again.

Then who will defeat Shanks?
Luffy said he will fight Shanks before, but he also said he will defeat Big mom as well after Kaido, so its not a reliable statement right now.
Is Shanks will be a part of the Government battle, perhaps even fight for the Government? Could be interesting.