Personally I don’t think there will be. There’s just Zoro and Sanji as Luffys left and right hand.
However, this begs questions about future matchups, there are 3 admirals under Akainu. I don’t agree with the “Fujitora won’t fight the straw hats” theory because that’s against his entire character.
Personally I think Usopp will be that next strongest character but not in the same way as Zoro and Sanji. Usopp will be the most notable other than these two and worth the most bounty, but his strength will come from his trickery.
Jinbe - Jinbe will be up there and I’m sure he’ll be a powerhouse, but I see him as fighting the CP0 leader and freeing slaves in mariejoa while the “commanders” fight the admirals.
Yamato - If they join the SHs they’ll be strong, but I wouldn’t place them over Jinbe and definitely not over Zo/San in terms of power. Plus Oda wouldn’t just add the fourth most notable straw hat to the crew at the end of the series when there are these characters he’s built up since the beginning.