How to make particle systems look more natural and realistic in Fusion?
Fusion doesn't have fluid dynamics simulation built in.  To get a realistic particle flow of a natural system, you'll need to use software specifically built for that task, import the results into Fusion, then use Fusion for compositing the smoke by adjusting the way each particle appears on screen and how they blend with the other scene elements.  SideFX Houdini with pyro, or Cinema 4D with X-particles are two pieces of software which might be better suited to fluid particle simulation. 

Most people don't think of fire or smoke as fluid because they're gaseous and aren't wet.  However, in the context of particle VFX, fire and smoke are fluids and require decent fluid dynamic simulation to appear natural.