Hey! So I tried FFIV Ultima, and I have some good things and some criticism.
Good things: I had fun throughout a the first 95% of the game. The changes to a lot of characters were welcome. Cecil being able to dual wield, Rydia being able to use harps, and Edge significantly buffed. Great changes.
The new bosses, starting at Mist Rage, were a welcome surprise and a lot of fun to fight.
Criticism: Starting at the end game, things started to go downhill pretty quickly.
I feel the optional bosses are a master class of what NOT to do to make post game bosses fun and challenging.
1. Many of them require grinding to be able to beat. That includes items and xp.
2. Many of them use random attacks, so bad RNG can wipe you, and that bad RNG is actually pretty common. So you need to get lucky to win.
3. Many require abuse of hidden in-game mechanics, such as lowering Cecil's agility, making him an 'agility anchor' as enemy agility is based on Cecil's agility.
4. There is a preferred order of super bosses, but no in-game hints on what to do.
5. I'm not a personal fan of what I call, 'map puzzles', where you have to figure out what combination of vehicles to use to get to a certain place, or if there is a secret path somewhere with no visual hints or NPC hints. For example, the hidden path to Goddess is fine because a crystal gives you a hint in the Core.
So really, in order to beat the endgame, you have to either figure out everything yourself through a lot of frustrating trial-and-error, or you have to ask on the Discord. Neither of which is especially fun.
Here's what I would do to fix things:
1. Exponentially increase the amount of experience given for fighting stuff in the Lunar Core. The goal is that by just fighting things as they come, you should hit level 99 by the time you reach Zeromus. This takes out the grinding requirement. Maybe increase the pink tail drop rate by a lot... like a 25% drop rate. Or better yet, make the bosses in the core drop pink tails 100% of the time.... so you don't have to grind at all.
2. All of the post game bosses need to be majorly revamped so that they present reasonable challenges, not insane 'need to exploit the game and hope for good RNG' challenges. The latter isn't a challenge, it's an exercise in frustration. It's fine to have 'puzzle fights' like Raiden, where the fight is easy once you figure out what to do... if there are hints in the game. But even that needs a bit of a tweak: Raise the amount of time between when he raises his sword, and does his Atom Edge. Make it so the Lunar Core revives his defender less often. That sort of thing. Other examples (this isn't exhaustive):
* Get rid of the RNG digest for the crystals. That just plain isn't fun.
* The enemies that have a 1HKO attack? Slow them down so people have time to actually use decoys to counter that. (or better yet, give Cecil a late game item that makes him auto-cover everyone, even if they aren't weak, and drastically increase his defense so he can survive multiple of those attacks; so his role of tank is actually a thing)
* Goddess is designed so that it's a cakewalk with charm protection, or impossible without. A better idea might be to give her an unblockable charm, that she uses far less often.
3. Provide much more in-game information on the post game via NPCs. This includes where to find the post game bosses, the order to fight them, and a good tactic to beat them. More examples peppered throughout the game to give you advice would be great. For example, an NPC in Baron or Fabul that says "Hey, headbands prevent charm, you might want to stock up on those!" Would be grand. The goal should be for an average person to find everything and beat your mod without any help from Discord.
All in all, I think your mod has potential and is nearly there, it just needs tweaks and such. Thank you, and good luck!