The Custom Action option is far too restrictive to be useful outside of the most specific cases. For example, you can't even make a custom spell that fits with typical spell rules -- your ability modifier still gets added to the damage! You shouldn't need to homebrew a whole spell just to add a simple "1d20+CHA to hit, 3d4 fire damage" action to your sheet.
The Custom Action should, ideally, apply to weapon attacks, spells, ability checks (for skills and for tools), and enable limited-use features. It should definitely allow for dice bonuses to single ability checks, such as the extra 1d4 for a check associated with a Dragonmark.
This would also be a great opportunity to support non-ability-dependent checks. For example, many tool proficiencies can be used with multiple different abilities, and this could support the "Skills with Different Abilities" variant rule. Ideally, the Custom Action could be set up to make a skill or tool check that allows you to select which ability score to use when making the check, every time you click it.
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