Background:
During the early development of electricity in the 19th century, all sorts of frequencies and voltages were used for various reasons. The result is that we don't have one worldwide standard for electric power at the mains.
However, what if during the early development, that via committee, market forces, or some other means, that there was one worldwide standard for electricity? You can take this to include the plug as well if you wish, though that may be a bit harder to make happen.
I wonder about the subtle impact this could have on economies, especially when electric use becomes more important in the 20th century. It could also have an impact on the inventors and their related companies. I wonder about what major changes this could induce down the road in world history.
For the sake of discussion let's say we start on 50hz and eventually reach ~230V as the voltage main electricity, with 3 phase being more prevalent (but not absolutely required) in areas that it would otherwise not be in when that comes along. Feel free to use a different setting as well if you think it would have a different impact.
One small shift I can already see: Japan would be able to better cope with the power situation after Fukushima as the entire country would be under one frequency.