The next base is going to be the Pirates base and they will be made of Polystone.
I will have a website online very soon that I hope will answer all your questions.
I expect to do Cars and the Incredibles at the same time.
Things will speed up when I have an actual Playset piece to work with.
It has been messy trying to workout the dimensions and details without having the items but I expect to be able to make bases in the future that will be ready before Playset and Figure release dates.