Man, thanks for the great and varied responses guys--one departing drastically from precedent and another doubling down on it.
Thanks for being a good sport, it's all just a creative exercise in the end.
My pleasure. The point is to foster a community of respect and discussion, after all.
I like the list of classes you ended with even though the class names weren't your focus--Warrior, Cleric, Shaman, Mage, Shadow, and Mystic--I feel like a lot of information is communicated by evocative names (my earlier comment about Shaman notwithstanding). "Defense-specialty Martial character" for example doesn't have quite the same oomf as defensive Warrior. Likewise, the MTG colors are super evocative with a lot of room for nuance.
Well, I would actually give
names to each of the specialties, I was using that phrasing to try (however lamely) to adhere at least to the
letter of the "only six classes" restriction rather than the true
spirit of it. Because, if you actually give all those things names and at least one unique mechanic apiece, I don't see much difference between "six classes, where five of them have four named specialties each" and "actually twenty classes all organized into five super-class groups, plus one weirdo omni-class."
Warriors: Fighter (offense), Guardian (defense), Warlord (support), Rogue (control)
Clerics: Avenger (offense), Paladin (defense), Priest (support), Invoker (control)
Shamans: Barbarian (offense), Werebeast (defense), Druid (support), Ranger (control)
Mages: Sorcerer (offense), Swordmage (defense), Bard (support), Wizard (control)
Shadows: Assassin (offense), Ninja (defense), Siren (support), Warlock (Control)
And then Mystic stands alone, having no/minimal formal "specialties" of any kind. Individual Mystic-specific abilities are thus likely stronger than individual abilities specific to other classes, but (as noted) these would be balanced against having to
build your synergy rather than starting with a cohesive foundation. Iterative testing becomes critical to ensure that the Mystic doesn't overshadow the others due to the
true "anything goes" flexibility it offers, as opposed to the "you have a semi-fixed starting point, but can grow in new directions on top of it" nature of the other "specialties."
Overall, the idea is to try to ameliorate the concerns voiced about the Source/Role dichotomy, and the perennial fears of "bloat," while still offering distinct structures and ideas. This would also (as much as I dislike this overall approach) likely welcome a "single list of Talents per Class" approach, where all Clerics draw from the same pool of Prayers, but individual Prayers may have special effects if you have certain class features or the like. E.g., the Power of Justice talent may have a core effect like punishing an enemy that has dealt damage, but this could have extra riders or tweaks if playing an Avenger or Paladin, because the former is all about hunting down and destroying heresy, while the latter is all about protecting and inspiring the faithful. Meanwhile, a Tactical Precision Warrior Talent might have different riders for Warlords (who use it to coordinate
others' precision) and Rogues (who use it to disrupt
enemy action). Pulling this off with
finesse would be a challenge, but it might be resolvable if individual "builds" (e.g. things like 4e "Artful Dodger" Rogues vs "Brutal Scoundrel" Rogues) are re-designed to hook in to Talents in some way, rather than Talents being designed to have build-specific riders--perhaps the use of keywords would be beneficial here.
There are also some other considerations. Warriors don't use "magic" in any meaningful sense, instead drawing on the beyond-Earth-physics grit, determination, perception, cleverness, and sheer plain
luck that pervades fantastical universes, and they to some extent share this affinity for physical solutions with Clerics and Shadows (the former often associated with heavy armor; the latter wear lighter armor, but heavily use arsenals of many different weapons). Shamans and Mages manipulate the physical elements, the former more in a geomantic/environmental kind of way, the latter in a "classical elements"/Wuxing kind of way, while the other three sources are much less elementally-inclined (Martial especially, where such Talents would be rare to nonexistent). Shamans and Clerics are strongly associated with healing, and Martial with preternatural endurance; it isn't that the others
can't heal, it's just not their strong suit. Shadow is the
heart of BS tricks that Get The Job Done, but that attitude is shared with Warriors (with their focus on tactics and forms) and with Arcane (magical esoterica). All five have the potential to interact with non-corporeal entities (in order starting with Martial: Legends, Divinities/Angels, Primal Spirits, Elementals, Demons/Devils; Mystics have their whole psionics/telepathy/sci-fi stuff going on so there's plenty of room for them to have their own Stuff in that field.)