Maybe you could be in the right way...
A few chapters ago, I thought that Imu and the five Gods would be the final villains but after Saturn's poor performance... I am not so sure about this.
The dude didn't even use a named attack/special ability, and he lacks for a combat/hybrid form.
At this pace, these guys are ass.
Then, we have the Holy Knights... But it doesn't makes so much sense that they would be stronger than the Gorosei because they would be the Gorosei and not ranked as Holy Knights.
Shonen logic, but Oda is playing with fire so I don't know anymore who will be the final villains.