It’s hard to come up with ideas without any context, ESPECIALLY nitty gritty. So instead I’ll try to start with a few frameworks that could be built upon. Maybe my ideas are bad. Maybe they are interesting. Everyone can judge for themselves. But I’m going for framework and inspiration here of how to start. I hope someone finds it interesting enough to take a thread and run with it.

You could have a magic system that’s based around breaking current physics which might feel grounded (like you know internally that a magic alters quantum wave functions and influences physical outcomes as a result for instance, or just stick with the good ol’ air earth water fire that people are already familiar with and will understand immediately). That would put limits on what your magic could or couldn’t do and make it feel plausible or at least consistent. It would also be easier to set a power level to since it’s already by nature restricted.

You could also choose more narrative inspired system which might be less cohesive but allow for bigger moments with your characters. Just imagine a cinematic moment. Your protagonist is on their last legs. The BBEG has hordes of minions overrunning the towns and cities. One of the protagonists friends embraces them and as tears fall the BBEG prepares to annihilate them. Call back to a scene where you foreshadow this moment and as the protag remembers that scene they find within themselves the strength repel the BBEG. Or maybe they can teleport away with their friend, or maybe they sacrifice themselves to destroy the BBEG in a metaphysical explosion. The specifics of that would be yours to craft but the moment you design would inform the ‘magic’ required. To make a system like this I would imagine a few different impactful moments that would be super fun to have in the story and their magical effects. Make these just about the top end of what magic can do in your world and then start scaling down to a more reasonable level. And now start thinking about how people would use the scaled down stuff in their day to day lives.

It might be interesting to allow for the balancing and OP-ness of the system to be restricted by sacrifice to the characters. Maybe there’s no real limit to magic aside from how much of yourself you’re willing to give up. Perhaps giving your entire body and soul could cause a magical effect of a certain power level. Maybe if two people cast it together, sacrificing both of themselves it would be more. The good side could quickly go down a dark path trying to out muscle the BBEG.

You could also go the route of taking inspiration from RPG systems like Shadowrun, Dungeons and Dragons, Call of Cthulhu. I’m definitely NOT saying copy those systems, but you could take from DND the concept of doing a ritual each day to be able to have access to certain stuff. Or maybe you like the idea of casting spells taking a physical toll on the caster as it does in Shadowrun. Or maybe accessing the arcane drives one made like in CoC.

So sorry I don’t think I can get into nitty gritty details but hopefully something in this sparks some inspiration. And if not, at least it can rule some stuff out!