For example, a game like Robot Ops would have been a perfect match for a Challenge Mode.

The Challenge Mode I'm thinking of is not "Hard Mode" - it's a series of challenges with specific requirements for beating a level. For example, beat the level while collecting only 1 token, or beat the boss while using Solar Razer exactly 3 times.

The game itself was originally concepted as a series of challenges, hence the "ops" in the title. But I ended up only adding 4 ops, with 3 per level. I could have easily added the Balloon Ghost balloon game as one type of mission. But I can also say that I could have added anything to the game - flying enemies that chase you and explode, falling platforms, portals, etc. 

In the end, a game is what it is. I can only move forward and learn from my past games. Challenge Mode might be something fun to add to a future game. Not Balloon Ghost, though - I'm going to keep the design on that pretty tight so that I can actually finish it!