I've been thinking that there should be a mod that focuses on one enemy type that will always move faster than you even with maxxed Running skill.

The only way to counter this enemy is to have line of sight towards them at all costs. Something similar to Weeping Angels from Dr. Who.
To add a curve with this, a blinking mechanic should be implemented that has it's own duration determined based on what type of moodles you have affecting the frequency for blinking.

Additional enemies that can be added with rather having an attacking purpose could simply be fakes meant to throw off the player such as, distant visual hallucinations that disappear if you get too close or you lose sight of them, an invisible enemy which just exists to produce rapid footstep movement around the player. Forcing him or her to stay on edge for something that doesn't exist, and an enemy doing a false attack from behind when they appear within the players hearing range.
These enemies should only appear or interact with the player if no other real enemy is within the area or if the player often finds him or herself confined to a closed environment.