Don't expect any theorycrafting, this will basically just be about visuals (which begs the question, why post it at all? Well... why not?)
With the addition of VPR, I'd say we've (unfortunately) exhausted more conventional types of swords, and while there's nothing stopping the devs from giving this Job a mace or hammer (I'd rather that went to a melee Healer, though. A Cleric or some such), you can't really call a "spellblade" that if there's no blade involved, can you? So the type of weapon I'm thinking of is... a pata*! More commonly referred to as a "gauntlet sword"!
Sketch I drew of how one such Magick/Mystic Knight might look
Here's a video on their history and use, for anyone curious! As far as video game examples go, the one that comes to mind to me personally is Raziel, from Legacy of Kain, though there's probably others! Dark Zealots from Starcraft, maybe? I think there's potential for some cool animations with the big, sweeping motions demonstrated (funnily enough) in both videos and, since weapon enchantment is a staple of this class (r-right...?), I gave that some thought as well. At least... as far a casting animation goes...
Personally I'd make it a DPS, so that it doesn't step on either PLD or DRK's toes as a third (fourth, if you count GNB) "magic swordsman" Tank, that uses Maiming gear, to distinguish it from both RDM and SAM.
That's... about it. Nothing substantial, really, but I figured I'd share regardless.
*I'm aware there are MNK weapons using the term, though only one model (and its variants) even remotely resembles the real weapon. That shouldn't be an issue given RPR exists despite scythes being available to a slew of other Jobs previously, I don't think, and the fact it would be a single, much larger, weapon used in a completely different fashion ought to make it distinct enough, I reckon.