I don't know what are the Conditions for Possessing someone like Imu did with Gunko,
But what's the point of Oda creating this Ability if He is not gonna turn few Characters We met before into Holy Knights?
Or Show us the Process being done in Present rather than the Past,
This is why i expect Imu to Create Two more Holy Knights, who will have a Rivalry with Luffy/Zoro,
We might see Holy Knights Kid/Killer very soon,