1.
Hydrochloric-Acid-Filled Piñatas
Good: Have the sturdy construction required to ensure no unintended leakage of contents.
Bad: Possible severe burning. Brings the party down.
2.
Endangered-Animal Piñatas
Good: Kids love animals. High potential for very cute-looking piñatas, like baby seals, for instance.
Bad: Beating with a stick sort of sends the wrong message.
3.
Particle-Accelerator Piñatas
Good: Built full-scale and often several miles in dimension. Therefore, young children find them easy to hit.
Bad: Each one worth several billion dollars. Parents generally not keen on damaging them.
4.
Smallpox (Variola major) Piñatas
Good: Cool virus shape.
Bad: Highly contagious and high mortality rate. Would also bring party down—as well as everyone else within a 100-mile radius.
5.
Infinity-Symbol Piñatas
Good: Possibly a way to address the often reported decline of mathematics education.
Bad: Thinking about infinity makes my head hurt. Now imagine having to explain it to a child over and over again.
6.
Piñatas in the Shape of the USA and Filled

With the Greenhouse Gas Carbon Dioxide
Good: Sort of work as a metaphor for the United States' role in the global-warming crisis.
Bad: Unfortunately, the irony would be totally wasted on a 5-year-old.