Okay, two simple and common theories I want you guys' thoughts on:
Gear 5th: Gear 5 might be Luffy using a staff in conjonction with his DF awakening to make it stretch and of course imbue it with Haki. It's a simple gear but can be stronger than his previous gears and might be the replacement for Gear 2 as his "normal" form, cause let's face it, he barely uses his Gear 1/East Blue form in big fights anymore.
Raftel environment: Someone, I believe it was Aravitz, mentioned how there could have been nuclear war during the Void Century. The WG doesn't anyone find out about it cause they could try to recreate nukes. What if Raftel has very strong radiation and that's the reason why Roger was sick? And it would explain why no one has completely explored the Grand Line as most probably died when getting near Raftel. I mean, why haven't any of the Yonko or Marines tried to get the One Piece? What obstacles are there that can block them?