We know BB, IMU, Luffy and Ancient weapons and Lily are linked with each other.
But when it comes to who will be final villain then it's not about BB or IMU per se but what actually happened during void century.
Oda has build this story on the basis of fate and destiny. He is drawing parallel between the past and the present.
Same thing will happen in case of FV. This brings me to believe that knowing there is connection between IMU, BB and Luffy and all three are kind of against each other then something similar happened in the past as well.
Ancestor of BB (Evil D) vs Joyboy (Good D) Vs IMU (WG).
So, I can draw two conclusions from it
1) BB definitely won't go down before Luffy or Robin reveals true history because BB has part to play in it or after it. BB is important piece in the history
2) BB vs Luffy vs IMU will happen - what I mean by this isn't triple threat but all three will definitely face each other.. their destinies are intertwined.