Imu believes:
1. Davy Jones will return through his Descendants
2. Joy Boy returned through Luffy
3. Lily might return through Vivi (Implied)
What's in common between these Three? They are all D.
It seems that Imu believes that OG D. can return through their Descendants/Families
In other words:
Original Monkey D.
Original Davy D.
Original Trafalgar D.
Original Nefertari D.
Original Jaguar D.
Original Gol D.
Original Portgas D.
Original Clou D.
However, what's Interesting is that for Luffy to be considered Joy Boy 2.0, He needed his DF,
Which is similar to what Blackbeard said when He told Ace his Darkness DF chose him,
Implying that these OG D. can return with their DFs,
We have so far:
Hito Hito No Mi - Nika
Yami Yami No Mi 
Ope Ope No Mi
Vivi will eat Lily's DF
Saul will eat a New DF
+ Oda will Reveal New Members of Gol Family, Portgas Family & Clou Family
Only when all of these Return, will WG Fall