You could also add Litecoin to the list, it has code that is very similar to Bitcoin and was used before as testing ground for Bitcoin.
Few months ago they also added MimbleWimble, and I think there are more coins that use this privacy method, but it never got more attention for some reason.
There is one Elliptic blog article explaining MimbleWimble privacy upgrade for Litecoin, and I am sure it wouldn't be hard to do the same thing for Bitcoin.
https://www.elliptic.co/blog/explaining-mimblewimble-the-privacy-upgrade-to-litecoin

I would always vote for adding any privacy based protocol change in Bitcoin but I am more than certain that would create huge conflicts of interest and probably hard fork.
Just look what is happening with shitereum now, exchange owners are saying they will support shitereumPoW, and they say they would shut down staking if threatened by regulators.
Imagine what would happen with Bitcoin privacy fork in similar scenario... if someone got threatened by regulators again.