In looking at
ACCUMULO-4375, I was a little frustrated at how we have 3x constructors than Key really provides just to support byte, Text, and CharSequence arguments. Additionally, the copy argument forces the user to use the most specific (most arguments) constructor if they want to avoid the copy. This makes constructing a Key from just a row while avoiding a copy very pedantic.
I think a KeyBuilder (or KeyFactory) class would be a big usability benefit and reduce the amount of code that clients have to write to most efficiently construct Keys.