Luffy and Shanks are not allies lol, best time people stop parading this narrative when Shanks said he was going after his fleet members.
Didn't claim they are right now. Said they would be at the end of the series, when Luffy is Pirate King and is finally coming for Imu. Shanks mirrors Law. Secret CD working for a D, Roger. Law was a secret D workign for CD Doffy. Just like Luffy and Law fought Doffy and Trebol, Shanks and Luffy will fight Imu and Garling. Perhaps Shanks just does 1v1 against his family member.
RHP will take care of Holy Knights
Revos will take on CP0 (Sabo vs Lucci)
Admirals will already have been beaten before the final battle (Akainu will get his own Arc)
So there is room to add CG to the WG forces. Zoro has to fight Mihawk. Usopp is a good match up for Buggy and so on.