No one ever said "I want Mihawk to be related to Imu"
I don't even believe Imu is Final Villain or Secondary Villain of Story
But Mihawk is a Knight-Looking Swordsman, Acts like Royalty, same Eyes as Imu, his Name "Dracule" is reference to both Devil & Dragon, and an Associate of Shanks & another Knight-Looking Guy who wants to Oppose WG & acts as Royalty as well (Crocodile)
It's obvious that there is a Shared Origin Story between all these Characters
But it's just Oda's way of tying Endgame Sub-Plots & Characters together
Personally, i believe certain Ancient Characters found a way to remain in World of Living by possessing People, just like Devil Powers are inside Fruits & get Recycled, those with Double-Layered Eyes are Hosts of such Characters
So Someone is Imprisoned inside Zunisha
Someone is Imprisoned inside Imu
Someone is Imprisoned inside Mihawk
Hakuba is Imprisoned inside Cavendish
And there are many more who aren't Revealed yet
Double-Layered Eyes appear when They reach "Awakening", meaning both are looking through those Eyes (Such as when Cavendish & Hakuba achieved Imperfect Fusion/Awakening), and the Host decides whether to give Control to them or not. Zunisha, Mihawk & Imu seem to be only Awakened Ones so far, with Imu even granting Control to that other Person, while Mihawk is waiting for a Swordsman who can push him to switch with other guy
Their Leader is probably the one Imprisoned inside Zunisha
Hence why Roger & Oden felt very uncomfy on Zou