How to make particle systems look more natural and realistic in Fusion? I have recently downloaded Fusion and have seen some preloaded uses of fire and smoke in reference to particle systems. However, no matter how much I try recreating something like that, it always looks fake as either the particles is too thick or it simply doesn't flow or doesn't look realistic. This applies to other concepts as well such as snow, fireworks, etc. What makes the particle systems look so natural in those cases? How can I achieve a natural more realistic look from them?
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.