I have multiple magic systems in mind, but I need one that would probably be the most stable and wouldn't allow overpowered solutions. Here the ones I have so far:
A magic system that runs on mana: Humans can't use mana, but objects can pull power from it. Humans are specially trained in handling mana objects. An object's potential can only be enabled by mana, which can be bought from traders, store-keepers, and merchants. Strength is determined by how rich you are, and how much mana you buy.
A magic system that tuns on magic crystals: Animals grow an extra organ "crystal" that allows them to use magic. Since they aren't sentient, they can barely use magic but humans kill and remove the crystals that turn them into watches, necklaces, rings, and other accessories. When a human has a crystal, it draws energy from the environment to do magic. Strength is determined by how many crystals you can get.
A magic system that runs on birthright: Every human is born into a specific magic domain and they can utilize magic from it to cast spells and such. Strength is determined by how much you train.
A magic system that runs on condensed energy: This one is more for a sci-fi setting, but humans have accidentally released condensed energy balls, which are floating balls that store a bit of cosmic energy. The balls act like living things, multiplying, and communicating but can be captured and used by humans. The amount of energy you can use per energy ball is based on mental constraint and physical ability. Strength is based on self-control, and how much you can capture.
Edit: Since there is a lot of confusion on what I mean I'll explain. By stable, I'm talking about a magic system that mostly allows improvement, and will not be biased towards certain users. This means the rich will not amass power and no one can be born with unlimited god-like powers. No 1, 2 and 3 might not seem like they fit, but I've also added the option of people who regulate the magic.