What I always envisioned as a good idea to promote diverse parties without outright enforcing it (which would not go so well) would be to have the drop rate for rewards on repeatable quests like Ifrit/Moogle increase if there is one of every DoW/DoM in the group of 8. Similar to the "everyone from the same Grand Company gets more EXP/Seals" idea, but with drop rates instead of EXP (or both, even). It doesn't have to be a huge increase, but enough to give people that temptation to try it. It'd be fairly simple to implement and non-intrusive for people that want to keep on doing 6 THMs or whatever it is this month.