They wont get rid of the class system, why? Because it DOES offer some advantages. For example the gun class they teased If they make it just Agent well... yeah.

But if they make it Musketeer and let it shoot into Agent like they teased possibly as the healer and pulled an Arcanist and let it also go a DPS road as a different job like Corsair or something like that it could still work fine. There is no real need to abolish classes, at all.

A Job like Dark Knight though they had the advantage of making something as a 2 handed user and likely they want people to level something new from level one (since many people would already level the class its based on to 50 and remove the leveling aspect) and give it brand new skills and abilities since if it was based off GLA, MRD or anything else for that matter. the skills would be redundant. It also gives them a good opportunity to bring a new weapon style into the game as well as more lore.