When the Straw Hats face the Marines, I see Luffy facing the Fleet Admiral then Zoro and Sanji taking on two of the Admirals below Akainu.
In most major arcs, Luffy takes on the strongest foe from a given enemy organisation and Zoro and Sanji take on the number two and three respectively. Some people doubted that Sanji would face the 3rd strongest Beast Pirate yet he still did. I have no worries about Sanji facing the 3rd strongest Admiral (including Akainu). I can see Yamato taking on the last one if there will still be that many on the Marines' side.
Facing the Yonko wasn't just about strength (that was obviously part of it but wasn't the only thing). The Scabbards faced Kaido to give this raid their all and attempt to make up for the past then the Rooftop 5 took over and faced the 2 Emperors to signify the New Generation vs the old. Sanji has no place there as he's an important member of the Straw Hats and the coming generation that way but he isn't a Supernova. He showed his worth by beating Queen and being recognised as one of the Pirate King's Wings (plus Marco made way for him and Zoro).
When it's time for Luffy to face the Marines, I'm pretty confident, Sanji will be given the 3rd strongest opponent that is encountered by the Straw Hats. Monster Trio and the Pirate King and his Wings has shown Oda sees Sanji as a major supporter of Luffy and even if it isn't a title based solely on strength, I feel like that's still part of it and it'll be reflected when match-ups for that arc come. I guess we'll see.