I already talked a little bit about this one with a new character I came up with, and it seems OP until you look at the costs of using it.
Semblance: Cosmic Singularity- the ability to use Dark Energy and Dark Matter as sources of raw power (Cosmics are not powerful on their own; they only become powerful when "borrowing" Dark Energy and Dark Matter from the universe). Resulting abilities- wormhole teleportation, small-scale black hole simulations (nowhere near as powerful as the real deal, and doesn't tear apart the fabric of space and time), directed energy "Quasar jets". If you have more than 15% of your aura left, you can use most of it to collect Dark Energy and convert that energy into aura to heal yourself.
*There are different kinds of cosmics, and all cosmic semblances are due to exposure to Dust mixed with Actinides (radioactive elements like Uranium, Actinium, and in the case of "Cosmic Singularity", Plutonium). Cosmic Singularity grants access to nearly all Cosmic abilities, but there are only two Cosmics who have ever had this semblance since Plutonium is an artificial element.
Costs: This semblance is the most powerful semblance, but if used without caution, it disturbs the equilibrium of the Universe itself, and as a result, the Universe fights back. If you're shooting out a Quasar jet and you put too much energy into it, it can bounce back and kill you. Reckless wormhole usage can obliterate you on the subatomic level and send particles of you flying to the ends of reality. If your aura is below 15% and you try to replenish it by spending it to gather Dark Energy, it'll kill you. Intense nightmares or an abundance of stored energy can make you highly radioactive, which could give you and those around you cancer (but, usually benign tumors).
In theory, Cosmics could be extremely more powerful than Ruby, but in reality Cosmics deal with some of the least conventional (by our standards) matter and energy in the universe, so the risks build up so fast that it vastly limits the range within which they can use their powers. For example, the new character I've been working on is Silas Nikos, Pyrrha's twin. Silas went all-out in a battle with both Adam (Blake's crazy ex with the red katana) and Cinder, shooting out two quasar jets at them. But, the jets backfired and hit him with 1.7 gigawatts of energy- that's the power of 8.8 Nimitz-class nuclear aircraft carriers! It's a miracle he survives!
Imagine it like this: In theory, Superman is ultra-OP, but in reality, to get that powerful he needs to eat Kryptonite for breakfast, lunch and dinner.