You could add a settings page
To controll options and attitude of your app to alow a more in depth experience and keep costomers involved and interested with this app.
Settings page could have....
Select the collor or collors of particles
tilt gravity options (on off)were u tilt it and all particles flow were u tilt it.
Maby you could add a slider controlling the life span if the particles were they dont just go away after a second, you could have them live n bounce around a while or somthing making cool designs.
Aswell as the distance or radious the particles fly And how violent they fly off or just speed.
Maby chose how they fly off,
so smooth arcs will look more like fireworks
Or zig zag particle trails so it looks like electricity shooting out your fingers and patterns.
An option were u could draw a line and the particles would constantly flow from ur drawing in a cool way.
So u could have a epic sparkly smily face or whatever my kid draws.
And with gravity tilt on u could make the sparkly smily grow a beard and cry or be on fire or somthing.
If you allow costomers to make drawings and have alot of different options to make it look cool, theyll sit there for a long time making different pics and doing different options to see how there drawing will act.