I'd bet they won't become monks. I'd say monk might be the/one of the advanced job(s) unlocked by PGL, just like thief also has a potential chance of coming from pugilist. I'm sure monks will dress accordly, using black belts, gis, shaolin robes, bandages or any other "martial arts" clothes they should need.
However, the correct is to assume that there will be PGL and monks, not being like those promotion trees some MMO use, because FF serie usually keep the previous classes/jobs even after new one was unlocked, which is very good IMO...i always prefered stuff being added than replaced.
