Due to Chopper's Devil Fruit Being The Hito-Hito No Mi (Human Fruit) it would make sense that his counter part would be someone who ate the Tonakai-Tonakai No Mi (Reindeer Fruit) So even though this isn't canon (Yet) Perhaps this is a way to have choppers devil fruit added without having to add a whole reindeer race XD, and rumble could still be a temp boost available to that fruit or doctor class.
-Kawaii