I was thinking of this earlier... What are ways that a fight can be created so as every type of role is helpful/useful and none need be left out in favor of "an optimal build", because having each role present - whatever the specific classes/jobs - *would* be the optimal approach.
Give every class/job a role to play in a given fight, and no one will have to be arbitrarily left out.
One example could be the necessity of close-up DD'ers to do some kind of specific damage to the mob which then weakens its defense against ranged DD... Or ranged (archers) using specific attacks to blind it temporarily so its accuracy is greatly reduced for a time; mob can't hit you if it can't see you. Or, perhaps have mages use certain spells at certain points in the fight to lower its resistance to certain types of attacks by all DD, ranged or otherwise,,etc. etc.
There could be a phase where the direct DD'ers aren't even really doing any damage to the boss themself, but are inflicting some kind of status or interrupt on it through their attacks which enable the mages to do their job more effectively... This way, they're still important in the fight, and it really doesn't matter if you're a PGL a LNC or even a RNG character... you can fulfill that role.
And to keep it from getting repetitive and boring, have the fight change up through different stages where each role is given a different task to fulfill in helping to bring it down. Instead of it following the same phases in sequential order, fight after fight, have it randomly phase from one to another and so the players have to be on their toes and pay attention when it happens.
There could be more strategy in terms of weakening the boss's defense/offense in order to decrease its damage and increase the damage it takes (besides debuff spells), instead of it being about "using the optimal party setup to do the most DD in order to kill it as quickly as possible". Devise ways that the fight is handled indirectly, instead of always through straight, direct damage dealing.
Remember how in Promyvions in XI, you had the different Animas that could be used at different points int he fight to help influence the boss's behavior? Something along those lines... but not exactly in that way.
This way, every role is important and every class has a use.