I don’t know if the World Government will ultimately lend Shanks some help or not but he will end up having help which will make the victory easy for him and he won’t have any losses on his side. The last time we saw Blackbeard, he was going after something that was connected to the Navy… What could have that been? An Ancient Weapon? Unlikely for Blackbeard to obtain something like that at this point. What he could be after might actually be Shichibukai who are getting abandoned by the World Government and hunted by the Navy. He is looking to absorb them into his crew and grow stronger.
What if they are after Mihawk who ends up joining hands with Shanks to crush the Blackbeard Pirates? That way both Shanks and Mihawk let the world know that the era belongs to them. Mihawk is just one man but he is enough to tilt the scales in the favor of the Red Hair Pirates and ensure a victory.
Now, let’s get back to Blackbeard Pirates being crushed and the parallels – it already happened in Impel Down when Magellan crushed the Blackbeard Pirates and just like in Impel Down, the Blackbeard Pirates will get their revenge which means they will end up crushing Shanks and his crew eventually.
You might ask how will Blackbeard Pirates go from being crushed to crushing the Red Hair Pirates? Thanks to Moria’s and Shiryu’s powers, they will probably escape death or being imprisoned and will return with another parallel – at the end of a great war, just like in Marineford, just this time it will be at the end of the war with World Government. That is probably when Blackbeard will kill Shanks paralleling Blackbeard killing Whitebeard. They will continue to secretly hunt for powerful Devil Fruits and make themselves stronger, many of them still have not eaten a devil fruit and some might obtain a second one by feeding their weapons to it, Funkfreed was one big foreshadowing.
Shanks will probably join hands with Luffy to take down the World Government after he learns about the Void Century.
Meanwhile, Blackbeard still has to reveal who his 10th Titanic Captain is and it is probably a big name.
A name worth keeping in secret. I have entertained the idea that it could be Weevil since he is another connection to Rocks which Blackbeard has too through the name of his ship and probably the same ambition of becoming King of the World while Weevil is connected through his mother who was on Rocks’ ship…
However, Weevil could appear at the end of Wano or in Elbaf as well and in that case, the 10th Titanic Captain would have to be someone else.
Anyway, Shanks and Blackbeard are two opposites, a day and night. Shanks will grab an incomplete victory for now and ultimately end up killed by Blackbeard during the War vs the World Government and that way, it will be Luffy who brings the dawn again and frees the world of the night that Blackbeard will try to engulf the world in.
*Theory by nik87