I think people think Sanji will face Greenbull and not Kizaru, because Kizaru after Aokiji's departure is theoretically the new second strongest admiral, leading to the idea that Kizaru will face Zoro (because Zoro is the second strongest blah blah) and not Sanji. We haven't seen Greenbull's powers yet, so I don't really care which one he's up against, but just the idea of Sanji taking on a fucking admiral is hype enough.