Theory I have on how Imu's devil fruit works and what gives him his power. 

His devil fruit gives him the ability to turn everything's and everyone's submission into his Omen. Everything and everyone in One Piece has a breath and a voice through the Breath of All Things and the Voice of All Things.

If the other 19 Kingdoms during the Void Century pledged their loyalty to Imu, they basically gave up themselves and all their kingdoms over to Imu's rule since he was the one with the Devils Fruit power and was the one who lead them all into the battles against Joyboy and the Ancient Kingdom.

 

The 19 kingdoms giving up everything they had to Imu (their life, liberty, and property) would surely give Imu a huge, world scaled amount of the submission energy that his devil fruit power turns into Omen. It's not only that it's only the 19 king's personal things, but it's also all of their citizens and their stuff, which makes how much energy and power they gave to Imu insane. 

He uses his ability to turn their abstract submissions into physical Omen and then makes the Omen do whatever he wants, whether it's making it explode, turning it into a long, extended sword (Stigma), making portals (The Abyss), corrupting people, marking people with the covenant to share different levels of his power, making himself giant with it, extending his arm to hit their opponent, or manifesting weapons with it. 

So his devil fruit's potential and scaling relies on his level of domination, which he pretty high since he's the ruler of the world.

This contrasts with how Luffy's fruit works. The Nika fruit is supposed to let the user bring complete freedom to all and give everyone their motivation and willpower back (their life, liberty, property, and the pursuit of happiness). 

Luffy's fruit works powers the user with their own imagination and brings the feeling of happiness and motivation to everyone around.

Imu's devil fruit also works with their imagination, but it's purpose is to oppress and submit others to their will, which won't ever let the person destined to use it have the same, powerful, and completely free imagination that the Nika fruit is destined to give someone.

I'm not saying this is his only ability, just one of them because I know he can transform, so his devil fruit it probably a zoan type.

Theory on Imu's power source