To me this sounds a lot like how WoW works with each class having multiple specialisations. I'm not saying that this is a bad thing at all, it's one of the things which makes that game attractive to a lot of people. And it is something I enjoyed while I played that game.
But I don't think FFXIV needs something like this. Every character can play every class and switch when they like. The game is designed so that it does not need classes to branch out into other classes, but instead that the class becomes more advanced when it becomes a job. The only real exception to this is sch/smn but SE have redesigned these jobs to have less overlap than before so their goal is quite clear; jobs are to be distinct instead of being merely different flavours of the same base class. I wager that if SE could turn back time they would not have arcanist be able to branch out to two jobs. They even said they regret this decision.
And personally I would far prefer to get actual new jobs instead of "hey here's another archer type job but instead of having a music theme we made it a sniper". Players already often complain about homogenisation and I think doing this could lead to more. Regardless of how a person feels about combat balance, I think most would agree they would prefer a brand new job with its own fantasy instead of introducing different interpretations of current jobs.
However this does not mean we should entirely throw this idea away. Going back to the archer, instead of having archer being able to branch out into multiple types of archers, SE could instead introduce a sniping job that uses crossbows. If they want to make it look even more different to bard and mch they could have the job dual wield two small crossbows. So we would still get a new type of ranged physical dps that uses projectiles, but it would be a job in its own right instead of just being something that branches out of an already existing job or class.
One of the reasons why multiple specialisations of one class work so well in WoW is because every character can only play one class. If you want to try a new class you have to make a new character. Having specialisations means players can experience different class styles on one character. FFXIV allows every character to play any job and switch between all of them whenever a player wishes. Which then makes the need for multiple specialisations within a job/class unnecessary.