Adding (and maybe answering @joseph’s question):
Some other 3D apps do have a “mix-material” where someone can use other materials to mix them by some controlling element (which can get really weird if you mix mixed materials and then mix them…).
So @Drvquiron’ answer is the best method to prepare some “to-be-mixed” materials or material presets (everything expect the Material Output node) as a group and reuse/ combine them into a new material.
Of course someone has to use the Mix Shader to mix different materials (or rather everything than the Material Output node to do so like i mentioned here:
and/or combined this as a re-usable group.