Well, as a die-hard fan of VI, i feel that it needs to exist - and alot of the stuff there cant be TOO complex. Mount can be done in Testing mode, and you can take control as an Adventure mode char in testinng mode to play as a mounted character. So all you would have to do in DFhack is to cause the same thing to happen, but with a button press. Otherwise, morphing would be a substitute, and in both cases we end up with working Magitek. Spells are entirely doable - im making a magic mod as it is. So I know i can make that work. And i know the magicite thing can be done - i just want an ideal temperature. Though i would like to make so that touching it temporarily gives you a summoning syndrome, and ingesting it, while it takes away your summoning abilities, gives you PERMANENT magic spells. I know of an Invisibility spell that would work for Phantom, Fireball would work for Ifrit, ive seen a few ice and lightning things too for Ramuh and Shiva. I don't know what all espers i woulld add, tbh. And I already know i could make the Phantom Train and similar things. If I want to make it weak to Holy, i create a substance called Holy that can be thrown like a Fireball for one of the spells, and give a material multiplier.
Not being able to give Kefka a tower by making him a megabeast sucks, but i could make the Warring Triad as Megabeasts, and make Kefka a type of very rare secret that the wielder cannot teach, and makes him extremely powerful, with all of the spell, and immortal. Make them have a OPPOSED_TO_HUMANITY thing, and they would gladly slaughter mankind. Which is perfect on that front.
Classes are best achieved through secrets or through castes - I would actually like to to it by making common liquid substances, brewed from common plants, that give a permanent Class syndrome. After all, some might just want to be a plain Half-Esper - or a plain Human with no special abilities except what they get by beating Espers. Actually, if Kefka wont be the only Megabeast, I can make the Warring Triad into megabeasts(as i said) as well as other major bosses, and leave the whole semi-megabeast category to normal Espers like Ramuh. I would also make Magicite an item used for a Magitek workshop that lets you create substances out of plants or something that, when ingested, give a Magitek ability to the user, such as the ability to turn into(ride around in) Magitek Armor, which would make them very durable and able to throw lightning, fire, and ice around. But no immortality or anything.
The only thing that I cannot think of a replacement for is the making townspeople dislike those with a magical syndrome. And yeah, it would be cool if there was a FF7 mod(FF9 im kinda meh on), but FF6 is my favorite, and I think a mod for it would be rad.