Zoro and Sanji fighting the same guy as a main arc opponent is a new concept. And the SHs have too much food on their plate even without Saturn's involvement. It makes no sense for Luffy not to fight an admiral after all the talk about fighting Yonko and the admirals. He's fought one Yonko and will fight at least one more, so it's only logical he'd have two admiral opponents with one of them being Akainu.
It's not like I am against Luffy fighting Saturn, but if the Gorosei are Im's OPest soldiers, then they should be getting beaten after the admirals. Saturn is here to take the Seraphim with him IMO. I don't think he'll be doing any heavy lifting in this arc besides showing us a glimpse of what the Gorosei are capable of.