1) Isn't it kinda late in the game to introduce a new sub-role? The devs have already confirmed they're trying to even out the ones we have; assuming the current 2 Job limit, they'd need to add a new melee for five out of the next eight expansions (or nothing but melee for at least the next two) to keep up, which would be extremely tiresome with 2 year gaps between each; it would
murder hype for expansions. Not to mention that melee DPS are already in the minority within raid comps, and this would increase competition for the slot.
2) Mechanically, what would a Melee Magical role even do differently than Physical Melee DPS? Ranged and Casters at least have distinct LB shapes from each other; Melee have a single-target attack, which is the big reason at least one is kept in raid comps, but it's hard to make a competitive side-grade to that. That's before getting into their role actions to boot...
3) Going back to the first point, the only job I can think of that would fit such a slot would be Rune Fencer (et al). There aren't many other jobs they could make to fill out such a role that aren't already represented by existing jobs like DRK, NIN and RDM.