FLUID DYNAMIC BRUSHES: Please build brushes like this

I'm glad you asked.

In the sample provided you can freeze the fluid simulation.

Additionally Imagine if you would:

A slider that would allow you to run the simulation backward or forward to a certain point to capture just the right moment.
Fluid Rakes (smilier to real world marbling techniques (yes painter has some of this in a primitive form but this approach would be much more realistic) but here you are working a more realistic fluid simulation)
Fluid Chemistry: ability to drop certain chemicals in the fluids (example hairspray, clear water, alcohol to produce different reactions.
Ability to adjust fluid viscosity with presets (example from a gaseous material, to ink clouds in water, to marbling like fluid, to highly viscous fluid like thick oil or silly putty.
Moisture: ability to work with clearly wet liquid to dry flaky material (example dried out, cracking paint) - think about how people working in arts and crafts might artificially apply a chemical on top of white paint to make it crack and age
pen pressure would impact how much pigment and particles you are seeding each simulation with and pen velocity and direction will cause the energetic effects - where you want the simulation to take your inputs and run forward until you have a result you are satisfied with.
You could also imagine this working with geometric rather than organic type seeds / particles (example squares, triangles, diamond shapes.