Shanks will spend a lifetime impotently losing every major head on war with Teach. He will eventually switch to tricky tactics to sneakily shank Teach while the latter is in a global war against 1/2 the remaining major global powers.
Teach will tire of Shanks's shit and give him a treaty to sod off forever. Shanks will accept out of practicality but will still grumble about the 1/4 of his territory that he didn't get back. The Red Hair Pirates fall into civil war regarding the treaty. Pro-treaty side win, Benn Beckman assassinated, more RHP dead from civil war than from fighting Teach.
The RHP and BBP will then become friendly, Shanks will start copying Teach's mannerisms and their kids will date each other.
This is not peak fiction.