When I first started thinking about what I wanted instead of Classes, my conclusion was that a Skill Tree was needed. The player would select a subset of the available root skills, and when advancing their character they would progress along those paths.

If they wanted their character to have magic abilities, they would select one of the handful of possible magic roots. There might be a root skill for patron/warlock type magic, another for wizard book learning based magic, another for earth/druid type magic, etc.

Similarly for martial abilities they would choose one root whose branches would allow them to progress in their fighting prowess.

They could have as many roots as they wanted, so long as they don't conflict with each other. Designing a character would mean selecting the roots that define their beginning character.

But having a wide-open set of skill trees while keeping it manageable and user-friendly is a huge challenge.

Then I saw what Daggerheart is doing with their system. While I think they really missed an opportunity to get rid of classes altogether, their making classes be mostly about the combination of two domains is a great compromise on skill trees.

By limiting a character to two domains, they are significantly reducing the complexity of managing skill trees while retaining the flexibility.

As I thought more about this, especially with how I was starting to think about combining Attributes into Aspects, it just seemed like a great way to go.