Again rounser, you are assuming the Mystic has any type of ingame reality. You can still call him a mage/druid or whatever. The mechanics would back you up on this.
I can rename the cleric a priest, too. That doesn't make the decision to use the Mystic Theurge name wise.
Specific flavor text for such a broad range of concepts as this class provides would be a waste of space.
If you gave it a real archetype which people know, such as Witch, there would be no need for such a waste of space, because people know what a witch archetype is. (I'm not implying that witch would fit, mind you.)
Mechanically this resembles the options allowed prior editions quite nicely. It must just be you who has the irrational need to add an organization or formal discipline of Mystics in your game....
Sorry, but this is bollocks. Every D&D class has at least a small archetypal basis, from the strong (wizard, thief, barbarian, paladin) to somewhat weaker (cleric, thief-acrobat, but even they drew upon strong archetypes such as the priest and cat burglar, if not perfectly mirror them).
The Mystic Theurge doesn't even suggest such an archetype with it's name. It's a backless maiden, a foil for a design artefact, with no purpose for existence other than what it's stats offer. The only class I can think of that comes close is the cleric, who was made good in combat (unlike how many folks view priests, and overlapping with the paladin in the holy warrior role, creating a D&Dism) so that the player would have something to do apart from heal. The less "clerics" in the game, the better.