Title, directional particles summoned by setting:

  effects:
    circle:
      - location: origin
        effectlib:
          class: Vortex
          particle: end_rod
          radius: 4
          particles: 20
          particle_count: 0
          particle_offset_x: 0
          particle_offset_y: 1
          particle_offset_z: 0
          particle_data: 1

Can only travel in the x, y and z axes. This severely limits what can be done with directional particles as of now.
https://imgur.com/a/IQswZDT

Solution:
To effectlib, add relative_particle_offset_x, relative_particle_offset_y and relative_particle_offset_z, functioning the same way as relative_offset, but for particles. The particle would travel in the appropriate direction, when particle_count: 0 and particle data is supported.

This would also allow us to have more control over the way particles are summoned when using particle_offset.

Edit: If possible, allowing auto-orient: true to cause directional particles to travel in the direction from the source to target would also be quite cool, and having a dedicated directional: true parameter instead of particle_count: 0

I'd like to third this!

Would also love this feature!