Man Zoro getting in Impel Down would be fucking epic. I rlly can't image how he will end up there tho
In Thriller Bark we had "Nothing Happened" Moment which was after Kuma said "I will show you Hell". Currently, we are gonna have 2nd "Nothing Happened" Moment due to Minks Drug so i believe Kuma gonna appear & send Zoro to Hell literally which is "Impel Down"
The reason that Place needs to be Re-Explored is because of Final Villains & Secret of Enies Lobby
According to my Analysis, there are Four Major Villains in Final Arc who will Ally against The World (Something far worse than Rocks)
Two of them are Im-Sama & Teach, Third One is "The Lurking Legend" & Fourth One is a Surprise.
The reason Straw Hats will be separated is because Oda cannot introduce these Four in same Location nor can he explore all Three Ancient Weapons Sub-Stories at same time, so there will be a Short Separation Arc where Main Players of Final War are properly Introduced.
As for how Zoro escapes Impel Down, Red-Hair Pirates will be there to save & escort him.