Good feat design?
Really though, the answer is design progress and completeness. PF1 was absolutely just nibbling around the edges of change, and at best cleaned up things like the weirdness of skill scaling and dead class levels, and doing strange things, like doubling the number of feats while roughly halving their power. If you wanted to take the design forward, you'd need to settle on which parts of 3e are core to the experience, and then double down on delivering them. My list of essentials is probably something like:
- Unified PC/NPC progression rules
- Completely explicated skills
- A robust object interaction engine
- Less a concern for me, but important to many people- Class levels as chunky point buy
After that, it gets a little wibblier on what's an improvement, exactly. Personally, I'd prefer to see more unique magic progression systems and better silo'd access to spells in general (more Dread Necromancers and Beguilers, less Archivists and Wizards). Throw in more aggressive skill effect scaling to higher levels, and I'd be pretty happy. I think core to the whole idea is that anything that's important should be modeled with specific mechanics, and that those mechanics should be transparent to players and available for their use.