Alternative leveling systems
I have played a couple of worlds, and I've realized that different leveling systems would be better fitting for different worlds. I think there should be a more customizable leveling system.
I think 2 new optional leveling systems would be popular and would find usage in most worlds.
Tier/ranks: instead of simple level numbers, you could have a level system tied to traits like F-rank to SSS-rank or cultivation realms, etc. Each level can upgrade the trait to a higher level. Also, rather than the current XP system, there could be a more customizable one that can be more specific to respective leveling methods, allowing cultivation worlds, or more specific types of leveling, like reputations, or even attribute leveling by doing things that would reasonably level it. The world maker would be able to define each level by name (from the trait), how to level it, what it affects, how many levels it can have, and would even allow multiple different simultaneous leveling mechanics.
Skills leveling attributes and attribute capping: This idea is technically two separate features. The first is that leveling a skill also levels attributes to a lesser degree. And the second is that skill caps and experience curves are determined by the attributes on which the skills are based. This would create a situation where you can only increase a skill if you have the capacity for it, and you increase your capacity by using the skill. This is ultra realistic and coherent to life. For example, you can only lift heavy weights by getting stronger, and you can increase your strength by training your body. Therefore, there is a feedback loop between a hypothetical skill of "weightlifting" and the hypothetical attribute "strength." If the skill weightlifting determines what level of weights you can lift, then it makes sense that its max skill level is capped by your strength attribute, but it also makes sense that using this skill would increase strength.