My opinion on Vampires.
Their power is directly connected to how they feed, and how close they are to humans/mortality. I believe that they feed on Spirit, which isn't the same as a persons soul. I believe when it comes to humans they are made up of three parts. Flesh, spirit, and soul. The Flesh is the anchor to reality, and the spirit is the tether to the soul which is completely outside reality. When the spirit is damaged, or weakened enough, the tether is lost, and the Soul travels on. If the Flesh/Body is rendered dead, the Soul has no anchor to Earth, and the spirit/tether is left behind as a shade/ghost.
I believe the Vampires use the body as a vector to the spirit to feed on.
Black Court: Most powerful. Feed on flesh/death. They kill a mortal whenever they feed, consuming all of the spirit. Completely inhuman
Red Court: Second to the Black Court. Feed on blood. Usually not lethal but very damaging to the spirit. Connected to humans, slightly
White Court: Weaker than Black & Red Courts. Feed on emotions through pleasure/fear/pain ect. Not too invasive when feeding most of the time. Much closer to humanity. I will note that I believe Lord Raith, at peak power may have been approaching a level that would push him closer to the Red Court. His kiss of death allowed him to devour an entire spirit, which I think is why he was so feared amongst his kind.
Jade Court: Feeds on breath. Not much is known about them. My opinion is they are similar to the White Court in power and connection to humanity. Slightly less human, or slightly more human.
I think the other Courts are all weaker. The Black Court, and Red Courts really are the strongest in that their way of feeding is most damaging to both the body, and spirit, and also the most efficient. Any other court that exists, I think, would be no stronger than the White Court, and their power level is determined by organization, reproduction, and collective cooperation.
I think that between the Black Court's personal power, and the Red & White Courts global footprint, there isn't much room for the weaker Courts. The White's and Red's sucked up all the power, and influence, and it's unlikely they would share it. So I think any other Court really doesn't have enough personal power, or ability to gain influence to matter much outside of maybe a small territory (like the Jade Court).
The Red Court is gone, but I think that power vacuum is being filled too quickly for the weaker courts to take advantage of it.
Other Courts feeding ideas:
Blue Court: Feeds on sleep. If they feed too heavily, the person dies in their sleep, and is seen as a natural death. Some people become aware they are being fed on, and experience sleep paralysis, and the feeling someone is on top of them. Science has explained this, but maybe not correctly
. Not terribly effective in combat because they require you to be asleep to be effective. If you are asleep, and they find you, you're in deep trouble. A side effect of their feeding is you wake up feeling exhausted, and making you more vulnerable to their feeding the next time you sleep.
Gray Court: Feeds on illness. They like to hover around hospitals, and nursing homes. They need the person to be weakened by illness, and then use that illness as a vector to the spirit. They prey on the weak, and suffering, making them hated by all mortals who know of them. They need to stay low because Wizards take great joy in eliminating them, and they aren't terribly powerful, weaker than the White Court, and not very effective against healthy people.