I would love it if Affinity Designer for iPad and Desktop would have a built-in emulation of traditional media such as watercolor which flows and mixes in real time similar to the real thing.
Currently there are a few apps/programs that can do this such as an Adobe project called Gemini which will have a paint/watercolor emulator for both iPad and desktop. A basic watercolor emulator in Adobe Sketch, and a very detailed one in Rebel 3 by escape motions.
A demo video of Rebel's watercolor in action.
A video of watercolor brushes being used in adobe sketch.
Adobes project Gemini iPad App in action.
Project Gemini's Watercolor Emulation in Action.