However, Abilities are an entirely different thing.
Technically you can already create custom abilities - but their effect is restricted to the mechanics already in the game. Entirely new Abilities require custom game logic that can't be data-packed. There may be ways how logic snippets could be prepared to be combined in an Ability's data file, but that would require major reworks of that entire mechanic from what I've seen, and it may introduce a lot more instabilities for an uncertain extend of extra customizability.
We're definitely looking into making as many aspects accessible via Data Pack as possible, but we can't make any promises, especially for the more complex features.