Hello! Just posting because I've never played a hack/mod of this extent before and I just want to say:I'm impressed. I've probably replayed FFIV a hundred times, so it was great to play it again but with so much new life. I hope you don't mind, but I'd like to offer some criticism, but know that it comes from a place of really respecting what you've done. I wouldn't bother to comment if I didn't think you had something great.
In terms of story/dialogue edits, I have no complaints but I do think you've missed an opportunity. I have always felt like the whole conceit of getting the sand ruby for Rosa comes out of nowhere because the game doesn't force your awareness of her sickness. Unless you explore the town (which obviously you should) you otherwise have no idea something happened to her. This has always bugged me. Also, once you save her, how she even got there is super unclear. I know she says she followed after but... how? When? It's blocked off! So I was kind of hoping you might take the opportunity to kind of fix that by maybe adding a bit of dialogue to the innkeeper as you leave, telling you about another person who needed urgent care, or something, and maybe show Rosa's sprite walking in from the left just before the Titan battle starts, you know? Like, she didn't make it to you, but she was there and that would explain how she got across the landslide. I feel so dumb for writing all of this, but it has legit bugged me for years! Along with that, consider some other small additions, like adding in quests like Yang's that solve your other characters' problems. A quest each for P&P, Edward, and Cid to get them back on their feet before the tower would be awesome. It's a chance to add to story without doing it a disservice. They all get healed, you just get to be the reason why and how they get healed, with, maybe, a silly/simple reward from each character for your efforts. Like a unique ring from each that enhances something related to that characters abilities. Like a ring called "Twin" that boosts the damage of Comet and Pyro, a ring called "Machinist" or something more character limit friendly that makes hammers work like Cid uses them from the front row, or a ring called "Bard" that enhances song potency or something. I don't know the difficulty of stuff like that, but they would be quality additions that wouldn't be in danger of threatening the integrity of the original storytelling.
Cecil: I wanted to address your goal of positioning Cecil as a tank instead of the hybrid DD/tank he originally is. I get where you were going, but I think it falls apart once you ramp up the difficulty. As has been noted here or there, your difficulty curve spikes dramatically once you get to the moon and especially once you start taking on the challenges. In the early game, Cecil-tank works well and makes sense. But later, he's just underwhelming. Not only is he weaker, he really can't stand up much better than anyone else either. In my opinion, if you're going to go the Cecil-tank route, he should be buffed in such a way that he's pretty much always the last man standing in an encounter. I feel like, with the best gear at the end, his defense should be maxed. Otherwise, give him some of his power back. It's so sad to see him end up a sturdier yet underwhelming white mage. Even Rosa hits harder!
Rosa: I also question your decision to add Edward's skills to Rydia. In the FF construct of powers it just doesn't make sense. It's very strange. You know what wouldn't be strange, though? Rosa having song. Pray is pretty pointless by the time you get her back from Golbez, so why not upgrade pray to song. Not only would it expand on a fairly useless slot, but it would mesh with the character better and you could make aim work with harps. I really think she's better structured for that ability. Otherwise, love the hammer add.
Rydia: On the other hand, what does make sense for Rydia, that you were wise to add? Charm! The difference between a beastmaster and a summoner has always been kind of semantic, but in FFIV, in particular, where regular beasts are actually summons, it super makes sense that one of the core abilities for a summoner would be charm. In fact, it makes so much sense that I found myself wishing it was an ability in her battle slot, replacing song, instead of a spell with a missing icon. In fact, generally, I'm not a fan of a lot of the added spells for her. I like the idea of the added imp summon (though I can't figure out where to get it from!), but I really feel like, if you're going to add spells to Rydia's summon list, they need to be summons complete with summon animations. It really undercuts the quality vibe you otherwise succeed in developing.
Edge: Great changes. Almost too powerful. There's no point to throw, after all, if his normal attacks do just as much damage! But mostly I think you've balanced this. What also works great is more of the magic additions for Edge. Even the spells from his youth. Even though the icons are different, because they're from his youth, and they look kid-like, it kind of works really well. I don't care for slap or Supernova for previously stated reasons. And thank you for giving value to steal! Fantastic! Too bad my inventory gets overloaded so quickly now.
Kain: Kain's white magic addition? No, dude. Kain is leagues away from white magic. Kain is leagues away from magic in general. Here, too, I think you had a missed opportunity to do something unique. What Kain is really missing is a AoE ability and something "Dragon" related. I think you should take some cues from other FFs and add a Breath ability (instead of white magic) that changes according to the weapon equipped (maybe element based? Like, self heal when holy is equipped, poison when earth is equipped, etc.?) and add a Sweep or Lance ability (instead of Yang's focus) that hits multiple targets but at 50 or 75% of base damage, kind of like the Sword ninjutsu. It would be super cool and on brand! Also, he doesn't have a lance to beat all other weapons for him (unless I missed it?). I know he can equip swords, but his best of best should always, always, always be a lance. God I'm pushy! lol
Otherwise, more generally, endgame stops feeling meaningful after a bit because it's just weapons musical chairs. I think some of the battles are clever, but the battles that rely on constant maintenance of non-duplicative spells (like blink, for instance) are really just annoying. I teach game design, I love so much of what you've done, but some of those battles... Bernard Suits says engaging with a game is the "voluntary attempt to overcome unnecessary obstacles." The more unnecessary the obstacles, the more you start to see through the conceit and start to read the machinations of the designer. I loved the solution to the second AI dude. It was unexpected yet obvious yet infuriating yet simple. But the Hyperion battle? Just tedious. It became obvious what to do, but doing it sucked and was boring. I always tell students to avoid making challenges out tedium. The only challenge to tedium is overcoming boredom.
Also, you add a lot of stuff, but you give very few hints about them. Those crystals were on point for the lunar stuff. And some of the secrets really should stay happy accidents rewarded for exploring (Edge's youth ninjutsu), but other things, I think, should be more explicit. I hesitated trading in those lunar crystals for a long, long time unsure that that was really what the dude wanted, and unsure that I shouldn't have saved them for some other purpose. Even after having traded them, I still felt it was possible they had another purpose. I don't think there's any particular value to leaving that vague and it's not super uncommon for you to end up with at least one of those crystals by the time you can make the trade anyway. So give us a bone and let us know things like that. On the other end of that is the hero's medal. Hero's medals usually aren't actual things you can do something with. But sometimes they're a type of currency. Really depends on the game. The first one of those I got, I figure it was the former, but when I received a second one, I wondered if it wasn't the latter. Maybe only make that drop on the first kill? Or add flavor text saying it's not worth or for anything? Or if it is (see, I'm doubting whether or not it's for something), add some flavor text when hovering on the item that might indicate where to go.
So there you go, a whole bunch of completely unsolicited advice. How pretentious of me! But I hope you're happy to receive it as it's in the spirit of absolutely loving your work. Well done!