Personally, i think Serephim are 100% off the table. Its been stated several times that they are not at their full potential yet (still children). Imo, the WG will get away with them by the end of Egghead.
Regarding Saturn, im pretty 50/50. On one hand, Luffy fighting a Gorosei would be pretty sweet. On the other, I still think Kizaru is the primary instigator this arc and will have a deeper connection to Vegapunk.
Regardless, I think Kizaru brought a "Buster Calls" worth of VAs. Essentially 5 of them. You also have G14s VA, so Im not sure if they'll be involved or not too.
So there will be 6-7 matchups at most. Luffy, Zoro, Sanji and Jinbe are probably all guaranteed. Hopefully Chopper, Brook and Usopp get some focus (they deserve it after Wano....)