I am more on planning making new abilities or skills. The concepts I have in mind for my characters can't be executed well using the available options, including the Heroic Tales Appendices. I am just afraid that if I make custom stuff, I end up making a very overpowered one.
Gotcha. I have a plan for the Character Expansion and game to update the current abilities to simply provide advantage. Giving advantage and a skill die is a little too powerful. It is much easier to just give advantage.
One other thing to consider is if something seems too powerful, you could reduce the max health to offset the power of the ability. Also, specializations can be something only that species can do. An example of that is the Linguist special ability in the Weird West supplement. They can "overcome communication barriers with area natives." That is something only the Linguist can do and doesn't provide any bonuses, it just happens.