Confidential Transactions on the Bitcoin blockchain at the protocol level:
https://people.xiph.org/~greg/confidential_values.txt
In this video, Trace Mayer claimed that this could have already been implemented into Bitcoin as a soft fork if everyone hadn't been so busy arguing and censoring people over the block size debate.
https://www.youtube.com/watch?v=uHXfEJD6DUk
I think he is right that if we can build strong privacy into Bitcoin, it becomes much more valuable to everyone, so the price and user base will increase.
(Protip: you can watch youtube videos at 1.25x,1.5x, or 2x to save time)