I have said many times I believe Egghead is an arc that mirrors both Sabaody Archipelago and Ohara. So, I think Luffy punching a Celestial Dragon is a given (maybe kicking him), but, I think Oda has already teased who's fighting Kizaru.
This was the point of showing Green Bull mopping up King and Queen: to make it clear that Zoro and Sanji aren't Admiral level yet.
Luffy matched Kaido with Gear 5, and dominated Lucci with it. It's time for him to get a "coming back to earth" moment.
Relying on the Seraphim, clones of other characters, to be the endgame villains for the WG is really lame, IMO. I think it's clear that the Gorosei ARE powerful, and we're going to see it here.
After looking invincible post-Wano, the Straw Hats will run into the TRUE STRONGEST FORCE in the series: the Marines/WG. If Saturn is Admiral or Admiral+ level, the Straw Hats are in for a bad time.
Making the Gorosei look weak right before the Final War Arc would completely destroy the hype for their characters.
What happened the last time the Straw Hats were full of themselves and fought with Celestial Dragons? They got destroyed, and had to be helped to escape. Zoro and Sanji will try to pay of their debts to Kuma by beating Kizaru, but, I think they'll fail; nobody on the Straw Hats can deal with Kizaru's speed, yet (Luffy in short bursts, but that's it).