They can't put the FFxi LB sys, coz they will need to revamp/redesign the whole LB sys and there is no elemental dmg balance sys (fire is weak to water etc), meaning there is no/not many mobs have elemental weakness. What they can do is to have 4 LB bars and 2 players can combine using 2 bars each to perform a combo LB with weapon dmg weakness as bonus dmg when done correctly. By weapon dmg weakness i meant the stat you see in your char stat screen that is always 0, blut resistance, slash resistance etc. I mean you see these debuffs in some bosses, like coil where enemies gets a buff of "slash resistance up" so why can't they take advantage of that. of course this will need some type of LB skill solely set on each jobs and are categorized based on the type of job you are, "DPS, Tank, Healer".