He is canonically one of Shao's top generals and killed Goro.
When did he kill Goro? He powered up with Blood Magic and cut his arms off, which Goro survived.
And yes "successor" in that clearly they were planning on having him take the throne, not literally as in Shao just appointed him.
Mileena ruled for about 17 years after Shao and she was clearly no Shao Kahn. Kotal ruled for about 5 years, and he never beat Mileena himself. Earlier Kotal fell out of Shao's favour and was given over to the flesh pits.
I just know what a power scale is.
People focus too much on "power scaling," as if that's something that's absolute.
Kitana spent the better part of 10,000 years assassinating people for Shao, after being trained by him,
and she never felt right about things and was never giving it her all. Doubt while serving under Shao was always a strong part of her modern character. While taking him on, she was in no doubt and absolutely giving it her all.
Wait, is this real? They really tried to make Raiden's and Fujin's role clearer, so to fix it they decided to call them a new tittle, and then backtracked and said they are not actually that tittle, making everything more confusing than it ever was? Not that I ever thought they were great writers, but dam, this is hilarious.
It speaks more about the fans than the writers, as many fans didn't understand the difference between Elder Gods and gods. So for
Mortal Kombat11, they started calling Raiden and Fujin "demi-gods," even though it's incorrect, to create more of a distinction.
Here's the interview, jump to 1:07: