Paintballers currently primarily use red dot sights to help with aim. Due to the trajectory of paintballs (more mortar than rifle) these are by definition a compromise. The dot will only be on target at one range.
A new type of paintball (First Strikes) offer increased range through improved areodynamics, however the trajectory/arc of these projectiles is still much more mortar than rifle. These rounds cost as much as $0.25 per shot and as a result optics are becoming much more important to players; but scopes, archery sights, bb/plinking red dots all fail to account for the arc of paintballs.
A camera app that when given range and type of round uses the accelerometer to prompt the player to move the gun to the correct elevation (angle).
Here's a thread
of folks already looking for this app.
At the most basic level this would be a cross hair projected over a video feed from the camera. The horizontal bar would be driven by the ballistics calculations and orientation relative to gravity.
Additional features that could spruce it up:
-range estimator similar to smart tools
-recording function to document all your best shots
-Here's the concept taken all the way to completion in the firearms world
I'm a mech. engineer and would also be happy to help with the ballistics tables if that is an issue.
It's not a huge market, pretty niche to be honest, but the folks who would use it would be VERY grateful, and unlike a lot of other ideas...there really isn't any competition for this one. The existing camera apps being used as stand-ins don't cut it.