I am okay with these sort of "growing into your subclass" sort of things but TBH in every game I've played that had it I can think of offhand it just felt kinda stupid and pointless. SWTOR is the most recent example I can think of. In a few cases the sub classes were really distinct i.e. Sith Assassin vs Sith Sorcerer but mostly it just felt like you had classes with shared similarities that only really added to the overall blandness on top of them already being copies of WoW classes.I think a more interesting setup for warriors would be to be able to don the leathers and be a skulking thief or strap on the metal and just be a mercenary like tank. I like the idea of following old class paradigms but it would be nice to see Pantheon change them up a bit.
Even in WoW with the way specs worked in the early days. You just ended up in a lot of cases with these characters that were bland and boring until they hit level 40 and got their spec defining ability (Mortal Strike anyone?). From a "fun" standpoint the change they made to let you JUMP into playing that spec at level 10 was pretty good. Even if it just added to the overall dumbing down of character growth.
What I'd rather see is classes that are fully defined and distinct with the "sub class" options just being flavor rather than being the actual "class". For instance a cleric in plate can tank a little and still heal pretty good but it's still a cleric. A cleric in cloth can't take quite the beating but they might be able to heal a little better. Rather than WoW spec trees where your priest is HOLY or DISC and can only heal or you spec SHADOW and can only really DPS worth a shit. Honestly I wish they'd just ditch that shit period and go with EQ or VG classes that are each wholly unique. I think letting a class fill multiple roles is a huge step toward bland and boring ass character classes as an end result. *shrug*