Cat is the prefix and what you want Erin Hunter to write about next is the suffix!
Cat:
For this one you can take your favorite cat and take the prefix of that! (Mine is Cinderpelt. So the prefix is Cinder-.)
New Series:
Birds: flight
Big cats: claw
Ocean creatures: tail
Any of the prey the warriors eat (voles, shrews, rabbits, fish): song
Other: heart
Comment the name you get! :D