I think there’s huge room for innovation here. I think a UI for creating PyMC3 models is a big task that requires some an incredible vision. In my experience, these tools almost always fall short.
A smaller, more achievable but similarly powerful approach would be to build a UI for configuring and running an existing model. For example, a UI that allows to switch out priors or set parameters for them, run inference, analyze outputs, all with just a few clicks.