That would make a lot of sense. I hadn't even connected the two but I think that's a good call. On the Katakuri note, I bet they'll just say he's holding down the fort and helping rebuild while Big Mom is away.
It'd take more for me to really start paying attention; but I'll keep an eye out. I was a Godzilla fan as a kid and both King and Orochi seem like they could fit Godzilla monsters now that you mention it. Godzilla fought a gold multi-headed dragon too. If that's a theme they run with, Mothra gotta get a rep. The two you mentioned, Mothra and Mecha Godzilla (if Oda somehow made this one it's a perfect opponent for Frankie) are the first four Godzilla creatures that I can think of from the throwbacks.