Nin in aiming gear would look garbage style wise imo...
A big chunk of their gear already is palette swapped aiming stuff.
-------
Regarding materia, maybe something like a materia version of a glamour plate? A materia plate, you bond it to your gear and then meld materia to the plate instead. Lift the plate off to open the slots, put the plate back on to reapply the materia.
Granted, it's a bit convoluted, but given FFXIV apparently has a rather awkward and convoluted way of storing item data I struggle to think of another way.
Just keep the same number of "Fighter" sets but make them shared by all physical DPS...and fuse STR/DEX stats into a single "Power" stat.
Technically, you can also put MND and INT into the same power stat, and have several "Mage" set shared between healers and magical DPS.
This way, you'll have multiple sets with different melds if you want to optimize several jobs, but if you only want to have them at level cap for casual play, you'd save a lot of armory slots.
I think they could put NIN together with MNK and SAM. They all wear cloth. Of course NIN has to be changed into a strenght job, but I don't see a problem with that.
DRG wears plate, so it would destroy his style to let him share the gear with the other meeles. But I would welcome a new meele job with a heavy weapon who will share the plate armor with DRG ^^
My opinion is somewhere between 'I would rather not' and 'Heck no!'. Not out for any practical reason, no, but simply because I don't like their usual choices for striking gear. And I'm entirely convinced that's the style they would stick to after the merge, so I'm not interested in a merge of any sort.
The current glamour system makes all issues with gear aesthetics a trivial complaint. It doesn't make any sense to have 3 different melee dps gear sets any more.