Controlling space, time, gravity and other energies could potentially make a character unstoppable. To add an element of danger and risk, you could make the magic demand a price for it's use. The more they use, the weaker they get. Or some spells / uses would cost more than others. The payment could be in the form of pain, physical injury, life force, loss of energy or overall magic potential - the more you use the less you have. Or the magic could totally weaken you, but be replenished with rest. Or the blood of lunch ladies.
Nothing is overpowered if you establish limitations. If you can go back in time and change anything you want that sounds overpowered. Add the stipulation that you can only use it once because you die 24hrs later... it becomes less powerful. You get one chance and that's it. Sprinkle some stress in and you may not make the best decisions.
Gravity is very interesting; it affects motion (energy), time, and space. I'm doing something similar about the derivative applications of gravity-powered interstellar engines. Nanomachines allow users to manipulate gravity with the energy stored in their bodies. Gravity manipulation is also used for non-lethal law enforcement, military, and general domestic applications. A neat trick to making science-fiction mechanics seem believable is to make the mechanics common in your established universe. One thing you should definitely keep in mind is the conservation of energy. That's one of the things that always bothers me in science fiction.
That's really interesting and I think you can definitely make it work. My sci-fi story has some of the same attributes as yours but mine are based off of the 4 forces of the universe. Space-time(Gravity), Strong Force, Weak Force, and Electromagnetism. So I have one character who is able to control space, gravity, and sort of time. I find that there are plenty of ways to avoid making it over powered, like having an equally powerful foes, or coming up with a bunch of rules on what they can and can't do with their abilities. It sounds like you are already doing that so you just need to make it consistent and restricted enough to not leave you open for plot holes.
Ex-human is what it is called. It's a very big story so they group of characters that gain these powers, don't do it until 2/3 into the novel. Maybe even a little more, but it is going to be at least two books, maybe three. The first 3 story arcs are 4 superhero origin stories. And also I'm still in the process of writing the first novel. The abilities range in difficulty so their is a progress to their power. A breakdown of their abilities would be: Space-time: Easy: Warping space Advanced: Creating black hole Electromagnetism: Easy: Magneto Advanced: Transmutation Weak Force: Easy: Become radioactive Advanced: Control fission reactors Strong Force: Easy: Take a good beating without much damage Advanced: Swim through lava They obviously all have a ton of potential abilities but some are harder to come up with. I think that is part of what makes it fun for me, pondering these things and figuring out how to make them work. If their is anything I've learned about writing it is that you can write about anything.
