It occurs to me that there's an opportunity here to appeal to Serif's apparent revenue model. Since the products are so cheap (for now), it may be that Serif is looking to make a platform play, where they develop a large-enough customer base to attract developers to an add-on market. We already see signs of that in their sales of textures, fonts, and brushes. What if they looked to open up these add-ons to other 'tools' as well? I could see access to an object model of the existing capabilities as important to this. e.g. I can imagine, on the low end, a plugin to 'sketchify' existing strokes to make them look hand-drawn, or on the high end, a 3d plugin where you can render a 3d model, possibly as part of a 3d animation package like Blender, in order to generate a 3d still or even a full 3d animation, modeled and animated outside but rendered by Designer or Paint.
In any case, a large part of this would be the development of an object model api that they would expose to outside developers. A scripting interface would be an incremental step beyond that that they might consider as a way to better publicize their capabilities to those who might be interested in developing such paid plugins for their store.
Anyway, just a thought.
-m