Core Concepts:

1. Extreme Versatility
2. Easy to learn
3. Complex, learnable, practical magic systems
4. Able host both a Medieval setting and a Sci Fi setting without needing home rules

Magic System Design:

These are one of my favorite aspects of Other to think about. I do have to say the premise of these magic systems have been shaped by my reading of the Cosmere, though you won't see any Surgebinders or Mistborn I bet some of you will be able to make them.

1. There are going to be too many
2. You can combine every system with each other to create things that I really regret giving access to

Leveling:

Your character earns Experience for a variety of reasons, which I'll probably go over later, and using Experience you can invest in a variety of feats. There are no classes, except for Magic Synergy Classes, but those are a can of worms I am too lazy to explain.

1. You do stuff
2. Get Experience
3. Pay some to learn a new skill or something like that

Combat:

Spellslinging, combo building, and attack dodging. Your turns remove your initiative until you lose a turn. They run fast(ish) and chaotically, every move could be your last. You should rely on creative attack design instead of overpowering force.

The first combo system I designed could make dealing 2000 damage too easy

Races:

Build your own race! Invest some points in features such as tough skin or natural psionics.