What I like about Akainu as a villain that he's basically Light Yagami in a way that he fully believes he's doing the right thing and sometimes, when marines do end up protecting civilians from bad/evil pirates, he is even justified in his PoV.
Imu is doing a bad thing, but he's so absorbed in his god complex that he simply doesn't see the world as a human does. And imo that makes him blander compared to BB and Akainu.
Both BB and Akainu are human, and measure the world as humans do. They are mortal, they have their strengths and weaknesses, they are more relatable.
BB is doing a bad thing and he knows it, he just doesn't care.
Akainu is doing a bad thing and he thinks he's doing the world a favour, and that's chilling.
Now, due to the fact that most of Akainu recent development as a character is off-panel, save for a few scenes where he's buried under the paperwork, we don't know how much he knows about the WG and empty throne, we don't know how he can justify the way CD behave, and this is something that I would find interesting to explore. Akainu turning against WG and still being a villain for Luffy would be very interesting to see.
And Imu? We know he's not going to change. He just has either to die or be chained to a chair like Aizen-sama.