I enjoy this idea very much.
Kurokikaze has a great idea as well.
I however enjoy (idea wise) not having to respec as Kurokikaze's idea works, if I wanted to play the other job.
Anyone in this game as well as in XI was able to choose which job to play whenever they wanted. Granted you had to have certain jobs unlocked, once you unlocked them you could choose what to play. All you had to do mostly was change some or all gear out.
The method expressed in this topic seems to have an advantage in that one doesn't "lose" the "basic" job in effort to get the more "advanced".
The affinity reference makes sense. It keeps the system of being able to equip other jobs abilities but has a built in feature of balancing them at the same time.
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Aside from unlocking extra jobs, we could just simply gain access to more jobs in patches or expansions. With either idea proposed by this thread or Kurokikaze's, one has to spend some time playing in order to get the other jobs. This is a built in benefit in order to "train" players at least some sort of skill set before they get to more advanced jobs.
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Either way, or a different way if the devs so choose, we will have to see additions to weapons if that is going to remain the way in which we choose our job.