Well, those sounds more like "minigames" than "Side missions".
Although, i would recommend:
- Rescue civilians: NPCs are being rapped through the map, go and free them before the enemies reaches the climax. (This might go with your idea of the new stage in the city)
- "On my way to doing your...": As a mob, you have to rush through a lane with obstacles to reach a woman and rape them before the other mobs. Randomly, you can be a horse, motobug or raptor at the beginning.
- "Sperm Invaders": Zeta is going to get pregnant, use a nano-bot to destroy the incoming sperms and save the egg. Depending of the kind of sperm, either animal, insect, robotic, gosh; it might be more difficult to destroy them (As a reward, you get one extra life)
- "Cha-Chaman Revolution": The Chamán from Lost World needs your help to find his 420, follow its indications (as a rhythm game) to go through a dark place, if done wrong the MC gets captured by a enemy. Once succeed, you get, a power up/Money.
Just a brainstorm, it's not necessary to follow everything that I said.
Edit: sorry for the cringe of my attempts of puns XD