Creating Dynamic Model Deformation?

What I'd like to do is basically the same thing seen in many modern games; face customization in Skyrim, muscle growth in Fable etc. Using Blender > Unity, I want to create two creatures, one an adult, and one a child that is basically just a simple model deformation of the original one. That part's easy, but then I'd like to set up a progression between the two that I can manipulate to simulate an aging process. So basically it slowly shifts from the child model to the adult model. I know I could do this with an animation, but I'm pretty sure that would preclude other animations from working alongside it; it wouldn't be just a base state of the creature. Does anyone know of a way that I could handle this? It'd be even better if I could add additional modifiers to the model at the same time, like more or less muscle and such.