Malkraz

Seeing as the Warrior had 6 skills in the initial build of the game before the secondaries hastily being removed, and the L1 skill trigger still being active within the games coding (hold L1 and it brings up its skill UI), I don't think much would be necessary in terms of a mod. Based on the presence of the L1 skill trigger, it's most likely the case (depending on how skills and vocations interact) that there are lines in the game's coding denoting what skill is placed into what skill slot. If this is the case, a modder could make a simple outside GUI that makes it easier to access these lines and select what skills to place in these slots. People have already managed to remove vocation restrictions on weapons (able to use greatswords to cast spells and whatnot) so I believe this demonstrates the handling of vocations/skills/weapons is not a complex system.