SJ and I have been talking about some new Rock Challenge ideas in chat a while back. So far we've agreed on a few ideas.
1. Air Shooter Challenge: Survive a specific number of Air Shooter attacks, then kill Airman. Due to the random nature of the attacks (some waves are undodgeable), 9-10 seems reasonable.
2. Pacifist challenge: Clear a stage while only killing those enemies you have to kill to progress. You only need to reach the boss door. So far I've tested Air Man, Quick Man and Metal Man's stages to see what you have to kill. Let's just say item 1 is recommended.
3. Quickman laser challenge. In this evil game of chicken you must touch a certain number of platforms in the second laser section.
4. Special weapon only challenges: Like buster only, but only using a specific special weapon. The weapon is usually selected because of its high ammo capacity. Exception: If the weapon of choice is useless against a mid boss or boss, you may substitute the buster.
5. Weapon restriction challenge (MMZ): Clear the stage while only using a specific weapon such as Shield Boomerang or a Rod.
Note: Normal difficulty is to be used whenever possible (NES normal for MM2). I'm probably forgetting a few challenges.