How can they add more classes and make them unique and have their own flavor?

[dinnertime]

NIN was probably the best of the 4, but from my perspective as someone who generally dislikes melee classes, I don't see a huge difference between DRG MNK and NIN.

Well, they do have some differences between each other.

MNK does highest damage out of the 3 melees. They have 3 utilities I think but One Ilm Punch it is not that useful and a couple more could be done by another job. MNK is notable for having a lot of specific positions for their weaponskills and that the skills depend on the form you are on and not what weaponskill you used previously that chains to another to make a combo. The weaponskill would be blacked out if you aren't on the correct form, so there's less chance of messing up your rotation from having your hands slip and pressing the wrong button.

DRG has seen relevance in challenging content with Battle Litany and Disembowel, it's arguably the 2nd highest melee DPS. Jumps are also powerful ranged damage.

NIN is pretty unique out of the three because of Mudras. It has the most useful utilities out of the other melees as well. I don't see the enmity-controlling abilities useful, but still handy. I'd say NIN does the weakest damage though.