One of the big things in polymer clay is making hollow forms. Hollow forms can be made using two part forms, air for the filler or dissolving filler. I’ve used all of these and I’ve used cornstarch “gum” to make shapes and cover them in clay to wash out later. The cornstarch mixture isn’t usually the most malleable stuff – seriously, it’s a great science experience, mix water and cornstarch – and it cracks as it dries. You can add glycerin to it to make it into a more useful molding material but after that it gets a little more complicated.
Oozeq is a new mix of molding paste made just for this: to create armature forms for hollow works such as polymer clay artists would use. There’s an Etsy shop you can buy some to use and a Kickstarter to help fund the start up project. Sounds like neat stuff!