@OP: my personal take on mystic knight/rune knight/parivir/spell fencer after a bit of thinking on it would be something like:
what would you like from it? - a warrior that uses magic via a more physical medium, such as spell-inscribed runes on a weapon, accessory, tattoo, etc. advancing in the class's quests could include learning and inscribing different runes from different magical disciplines onto your blade while picking up lore about their various regional/historical origins (so that your character can better encapsulate that magic through their deeper understanding of it's original conception and applications).
i personally wouldnt tie this to a specific region as an opportunity to touch on some otherwise abandoned regional lore from across eorzea and other countries, such as learning about the arts of geomancy while learning the principles of "runic aero/stone", or the nymians when learning "runic protect", the mhachi with "runic fire/flare" and the amdapori with "runic cure/holy" and so on (or the ascians with 'runic ultima' for your limit break animation, but that spell will never actually be allowed into the game for players).
...of course this is irrelevant since it would only have class quests and relevant lore for the initial 10 levels and then BaCk tO RoLe qUeStS WiTh yOu.
what weapon do you think it would use? - a hand-and-a-half (rune/knight)sword and an offhand similarly engraved gauntlet/bracer/bangle/hand-and/or-wrist-accessory (both for varied glam options and to actually bring that back--they already have the "pair the shield when the sword drops" code in for PLD, so why not run with it and make more use of the off-hand tab in the armoury chest at the same time)
which armor set would it wear? - maiming gear -- or alternatively go for the eastern 'parivir' route and have it use scouting gear (so something else finally uses that)
what would it's gameplay be like? - a class that uses a singular resource (we'll call it "runic charge" for example) in flexible ways--your bread-and-butter melee combo skills and some OGCDs charging it up, and all of it's "spells" are used with this resource, with each granting various unique benefits to disincentivise simply spamming the strongest option.
sort of a reverse of RDM for the most part: a primarily melee approach with physical attacks and imbued attacks for point-blank spells (and traits to cast without interruption), before dashing out for a big finisher--something powerful that they couldn't safely fire off point blank (could be referential for lesser-used "big spells" across the series, or sword arts like Stock Break and Shock from steiner/beatrice's repertoires)--and then back in to begin again.
my take would have GCD combos giving a bonus to the next offensive OGCD used, and OGCDs giving a charge-based bonuses to the next few GCDs (an existing example of this type of mechanic would be similar to PLD's sword oath stacks, or SAM's meikyo shisui affecting only the next 3 GCDs used)
the benefits granted and number of charges would vary between skills; things like GCD combo A increasing the next OGCD's potency, combo B decreasing it's cooldown time, or the aoe combo making the next OGCD instant-cast, as a few possible examples. OGCDs might increase the next ~3 or so GCD's potency, make the next GCD heal you (runic cure?), or other effects. the meditation/riddle of earth/manawall-equivalent could be "runic protect" to touch back on our old friend-spell or somesuch.
which npc would represent it - thancred, having the runes and attached spells engraved with the aid of y'shtola and urianger, as with the way it "charges" letting him act more flexibly in a protracted encounter or while on solo missions, unlike the GNB's aether shells' (narratively) limited amount if he doesn't have someone on hand to charge more for him, as well as (for a unique Trust-NPC-only-thing that he can do, like y'shtola's whirlwind spells as a BLM, or urianger's aetherial manip) executing dual-techs with any casters who might be teaming up with him akin to steiner and vivi from FF9 or for action cutscenes during the MSQ or something.