This will sound crazy, but it would be fun to have a map with a fixed start/end and groups of zombie at set intervals. When you enter the zombie area, you have to go harder depending on how many nasty zombies there are and how fast they are. If you go too slow, the zombies smack you and you go slower. To avoid getting eaten, you’d have to push harder to escape that area.
Getting hit by zombies would affect your speed for the whole workout or for a set amount of time.
It would make doing intervals more and feel less repetitive.