A super-compact global power adapter
Those who travel on trips through many countries face the problem of how to plug in their laptops and gear. Many stores sell collections of adapters, but they are often bulky, and having multiple adapters for multiple gear can be really bulky. (Usually you get one adapter and then use a 3-way splitter or cord for your type of plug.)
Today, however, almost all my travel gear is 2-prong, not 3-prong. It's mostly my laptop and various chargers for cameras, phones etc. And all of it runs on every voltage and hz found in the world.
It seems if you're willing to break the rules on rigidity of plugs, one could make a very small adapter by using independent pins, perhaps with a flexible rubber strip handle between them to keep them together and make it safer, but still allowing the pins to bend and have different spacing.
If you do this, there are really just a few types of pins you need. Thin blades, thick blades, thin round pins and in a few places fat round pins. The blades come at different angles -- parallel in North America, slanted in Australia, colinear for thick blades in UK. With pins it's more a question of spacing than angles. A single plug with a way to adjust the spacing could also work. (Israel has a strange pin I haven't used, I don't know if other pins or blades could be adapted to it.)
Generally this would not be suitable for plugging a wall-wart into a wall, you would want to plug in a short extension cord with multiple sockets of "your" type. And it might be hard to sell a product like this due to safety standards, since they don't want to trust the user to know what they are doing, know that they are only plugging in equipment that takes any voltage and doesn't care what pin is live and which is neutral, doesn't need ground and doesn't draw lots of current in any event. But it would be very compact.