it certainly can be done. it just doesn't make a lot of sense from a development standpoint. adding a new mechanic is complex and time consuming. it should be avoided whenever possible. That being said. we have a mechanic that can fill the same role. it fixes the current mechanic, and accomplishes in adding something new at the same time. this is a more ideal path forward.
also, because so much time and resources were put into the sytem already. the core will likely be the same. currently there isn't really a way to create specialization within the class without a complete overhaul of the system. in that case it's easier to start with a new project rather than fix a broken one. if you work for a company that builds refridgerators and you have a condensation pump that causes problems in production and fails too soon in the field; someone who knows nothing about manufacturing might say 'get a better pump to replace the bad one.' but it's not that simple. and often times you can't just replace the pump, because of logistics, proceedures, you have to re-train field operations, supply chain will also have issues cutting off one vendor and adding another. so 9/10 you're going to make a new product with the new pump and obsolete the old one and add a new tag to the product line. the same is true for software and games. it's already released. they can change a lot of things, but a complete overhaul is highly unlikely.
they will most likely use the existing system with some changes. it may feel very different from the UI, but the core basics will be the same. only time will tell.
specialization within a class should be done via guild marks IMO. not with a talent tree. you're creating a new system on top of a system that can acomplish the same thing. it doesn't do like you've invisioned, but this option is a bit problematic. mainly because specializatation within a class is a new class.
the current system of changing class (which isn't broken) is weapon=class. that basically means. if i equip a sword with dark-knight specialization. i'm a glad + more offensive abilities. which isn't really a new class. you're just calling it that. it'd be like concussive blow II. 'hey, i got a new attack.' except no, not really. it's just an upgrade of an attack you already had. there's no real uniqueness there.
INSTEAD: why don't we add a similar, but unique, set of specialization skills purchasable via guild marks for glad to make them 1) tank better or 2) deal damage better. or both at the same time, provided you have enough action points to put it on your bar and enough marks to buy both sets of abilities.
with this, you can then make pld an advanced class, unlockable, which can in turn have further specialization within itself. say phys tanking or magic tanking. it's less limiting with glad as a 'base' SAM could have guild mark specialization to focus on Bowmanship or Swordmanship. but still be different from dark knight or archer.
Rather than 'oh, paladin is an upgraded tank glad' or 'DRK is an upgraded damage glad.' it feels too gimicky to me. if you really want specialization AND class uniqueness, you can't mix them together.