I'll be honest: if this is your first time designing a tabletop system (not saying it is), try to take something that's adaptable to different settings and run with it.

I say this because designing a system from the ground up can be really, really rough. At the very least, you should have some experience tinkering with a system before you try it. Trust me, I've been down that road and know people that have been down that road. It's not easy.

Assuming you tinker with a system first, I'd suggest taking a gander at three systems: Dungeon World, Fate Accelerated, and Fantasy Age. My reasoning is that the first two systems have really simple mechanics and can be adapted to whatever you need (Dungeon World could easily be turned into a space opera if you wanted, and Fate Accelerated is just designed to fit ANYTHING). Fantasy Age... is just a really good system that already is coming in different flavors. It's based on the Dragon Age setting, has a more sci-fi oriented fantasy setting, also is coming (has already come?) in romantic fantasy flavor, and... It's really not that complicated, and it's one of the most fun systems I've used.

There's another reason I'm suggesting this: studying other systems lets you find out what pieces they have that you might want for your own. Do you want to use a d20 system? 3d6? Dice pool? Card-based system? Fate dice? There's a lot of different things out there you can test out.

[b]TL;DR:[/b] Study other systems before you make your own, and I do mean STUDY them. Figure out what you want from your Destiny-based game. Then do magic.

And to be honest? Balance isn't that important if you're just running this game for a small group of friends. If you wanted to actually publish this system, that'd be different; but if this is for a forum RP or for a table of IRL friends, you have the almighty power of GM Fiat to keep powergaming under control. For now, just make a system.