Sounds like the design vision behind Pathfinder Archetypes. I like the idea and hopefully they can pull it off in a balanced, precise way such that each subclass is legitimized. They will, of course have to deal with micro-balance functionality with respect to sub-class features given the potential interactions with a la carte multiclassing. Their statement of
"buy into completely or not at all"
may be difficult to square depending on how the multiclass system works itself out. Further, if you buy into a subclass "not at all", what does that mean...a stock feature instead?
I do particularly like their rendering of the "essential Fighter" as
"For example, we know we want Fighters to be extremely good with weapons of all kinds; we know they should be versatile in combat, and they should be among the most resilient and defensive of the classes."
However, for this to be legitimized, you're going to have to have Fighters either actually have a reason to use multiple weapons (a la carte effects/riders triggered upon weapon used) or the ability to leverage multiple styles/martial arts rather than just one; eg extreme mobility/skirmishing coupled with heavy melee control and (rather than vs) AoE berserking and (rather than vs) defensive stalwart who is a wall unto himself. This is one thing I loved about the 4e fighter in play. He defaulted to a focused chassis, but one that was "and" with respect to all of the above rather than "vs" and could be specialized or broadened further to your taste.
The reinforcement of the necessary non-combat functionality (skills or proficiencies) of the Fighter is heartening; eg more than "press attack buttan and then press drink, wench and crush tankard buttan".
If you can truly insert a
considerable number of subclass features onto each the chassis of each class, then they should be able to realize many of the classes with an intensely robust subclass system. If its "unplug horsie mount and plug in stag" and "unplug divine spells and plug in druid spells" to get a Warden out of a Paladin, then there will assuredly be something rotten in Denmark.