I personally(and this is just my opinion) Don't think the new class system or job system or whatever you want to call it should be built on the back of the class=weapon system. It just would not work in the long run. The idea of making new weapons subsets of currently existing ones is interesting but at the end of the day would prevent newer jobs from being added(take for example Bard. It would need a instrument and making it the class=weapon way would make it need all the bard songs to have enough abilities to be called a bard and then makes the title part impossible as those songs are already used....if that makes any sense).
I also don't like the weapon sub set idea because certain classes(GLD for example) would have far more subsets then others. For example. GLD would be sword and under that could fall Greatsword, Katana, Great Katana and Dagger(and I could be forgetting stuff) While something like say MRD would have just Axe(as the current kind is a great axe).
I personally think the best way to go about it is to abandon the current system and give the players a equivalent level in the new system or to have the current system just be the gate way into the new system and the new system will not rely on any of the older systems rules once the player gets access to it(which would be a bit confusing so this would be the worst case scenario thing).
I'm not very good at explaining things so I would not be surprised if this comes off as making no sense. But keeping what we have now(which is somewhat in depth to begin with) and adding an even more in depth system on top of that(one that players would HAVE to understand) would probably alienate players. It also would make it difficult to add certain jobs into the game and make adding new ones that doesn't fall within it's rules near impossible(this is why FFXI was so great. BLU had all new rules when it came out compared to other mages and the game allowed that. The only 2 basic rules were job and sub job how those jobs worked could be completely different in every way if they wanted to). The game needs to have a new system that is extremely flexible and not overly cumbersome. Because of that I think it needs to be rebuilt from the ground up and not built upon.(Sorry if that made no sense but I can't really get my thoughts into words in this case very well)