real anonymity. This is pretty much a requirement for fungibility, and for privacy from family, neighbors, big brother and big, inc.
It could mean coinjoin built-in and enabled for every transaction by default. Well the implementation is up to you, but the key thing is that it must be on by default, not some additional geeky step that nobody ever does.
If there is ever a coin that gets this right, it could be the one to overtake bitcoin.
Now a simple question, what would you see in Bitcoin 3