I love the FFIV Scenario Guide and I think there's a lot of beta ideas in there that never got off the ground and some story elements that were abandoned for better or worse.
Let's start from the beginning...
"Long, long ago...
a great light appeared in the sky.
It bathed the earth below in its warm luminance
and then disappeared for parts unknown.
Afterwards, four small lights remained
and a new planet was born in the heavens.
An evil will yet reached out for Earth,
but it remained sealed away by a strong power."
Quite a bit different than anything we see in-game! What is interesting is that this coincides with FFIV:TAY's overall plot very well... now if they would have Included this Anywhere in either game maybe the Final Boss wouldn't have seemingly come out of nowhere. Also I have to wonder at this point if Zemus was even thought up. Mainly because "An evil will yet reached out for Earth" I don't think the Lunarians looking for a new home is an "Evil will" that reached out for Earth.
"Tella <-> Rydia: teacher-pupil relationship, eventually" Here would have been a fun relationship to develop. It seems they originally intended for Tellah and Rydia to be a student-teacher relationship. The two never once talk to one another (he does at least speak to Cecil about her). I think this was a missed opportunity.
"Warriors of Destruction Who Fight with Their Own Life Force and the Powers of Darkness
The mightiest division of the Baronian Army - and one of the divisions the rank-and-file soldiers look up to and yearn to join - is the Dark Knight Corps."
Does it strike anyone else as strange that you never fight one Dark Knight throughout the game? There's a Dark Tree a Dark Imp, but no plain Dark Knight. They would have been an interesting mini-boss somewhere. (Dark Knights turned by Golbez I would imagine could be extremely dangerous foes).
All of Kain's backstory is not in Any game, and its a real shame. It's the only mention of Dragons in the plot of FFIV in the first place, they mention him being the son of a great Dragoon, but they don't expand that. Apparently his father was the master of the last trained dragon, but the dragon eventually died while under Kain's care (of old age) and it seems like the Dragoon was supposed to have a special affinity to Dragons. None of that carries over to the game.
"Wind Spear
It's said that the wind is the best friend of those who make the skies their battlefield. Therefore, the Dragon Knights are very familiar with the handling of this spear and wield it expertly. It's said to display formidable power in mid-air battle.
Blood Lance
Has the power both to stab like a spear and to cleave like an axe. Since the spear can siphon the life force from its victim's blood, its wielder can use it to heal his wounds by striking at monsters.
Flying Dragon Lance
Infused with the spirit of the nature-loving silver dragon; the spear still has the power to call forth his spirit at times. The spear is greatly effective against dragons."
Is it just me or do the Wind Spear and the Flying Dragon Lance sound like they were going to use an FFV-like Magic Weapon effect originally? Especially with the mention of the Flying Dragon Lance being able to call forth the spirit of the Silver Dragon.
Also, that description of the Blood Lance... yet its still the most worthless weapon in the game.
"Yoichi's Bow
A Japanese-style bow said to have been used by the hero Yoichi. [Note: the Yoichi referred to here is Nasu no Yoichi, a Genji archer in the Genji-Heike clan wars of the twelfth century famed for shooting a sacred fan pinned to the mast of an enemy sailing ship in stormy waters from a distance of about 70 meters at the battle of Yashima. Further info is available here and here.] Its accuracy is mediocre, but it's powerful enough to fell an enemy with a single volley. It also is said to possess strange powers that decrease attack power but increase the power of other equipment.
Elfin Bow
A wooden bow infused with elfin magic; it boosts one's agility and wisdom when equipped. It possesses great defensive powers against magic and is very effective against magic-user types. When used as an item, it emits a burst of magic power."
Whoa, whoa? What is this about Yoichi's Bow that decreases its attack power but boosts ?other equipment? how was that even supposed to work?
Elfin Bow is interesting, mainly because it doesn't raise Agility. To its credit it does cast Shell as implied here.
"Albeit Simple Tools, Splendid Weapons in Cid's Hands
Hammers are just hammers, and in the hands of everyone but Cid, their only use is as tools.
Cid, however, with his knowledge of machines, can use these hammers as weapons. He can instantly analyze a machine's structure; since he can discover all one's weak points in a single shot, it would be advisable to use them to deliver a sudden, surprise blow to the enemy. Of course, he can also exercise this ability on non-machine-type, organic enemies.
Earth Hammer
Fashioned from a special metal; the friction produced when one attacks with it creates an electric charge. It's especially effective against enemies weak against thunder. Extremely weighty; also endowed with a special feature that is said to increase one's power when equipped.
Mithril Hammer
A hammer generally preferred by artisans; also a mass-produced hammer. Cid also uses it to conduct repairs on the airships. They're advertised as being strong and sturdy, but they're not promoted as potential weapons."
Yep... they Sure are great against Machine enemies. How about all those machine enemies you fight while Cid is in the party? All none of them...
The description of the Gaia Hammer is interesting. Most of it is correct, except it is Fire Elemental, rather than Thunder as the description implies.
I love the Mythril Hammer's description because Even the game is saying "Its garbage, don't buy it."
"Various small groups of Summoners are scattered hither and yon throughout the world, but their bloodlines are weak and abilities limited." Other summoners? Well that could be an interesting idea.
"The Summoners of Mist have continually married within their own bloodlines, but that has certainly shortened their lifespans, and the continuation of those bloodlines looks doubtful. In any case, the future of the Summoners does not look bright."
Yikes...! I forgot about that rather creepy detail.
"Red Dragon
A frequently-encountered monster that lives all throughout the world, a dragon that has huge wings. Attacks enemies while freewheeling about the sky. Marked by its pure red body, its strongest weapon is its fire-breath attack, and so it is popularly called - and feared as - the "fire dragon" due to its alleged ability to command fire."
Or... y'know... nowhere In the World itself, but in the Moon, definitely.
White Dragon
"A mystic female dragon. It deals out extreme amounts of violence with direct attacks that make full use of its colorful breath and tail. In particular, the destructive power of its tail, swung with all its might, is said to be able to level an entire mountain in a single blow."
Is this the Mist Dragon? Because the Mist Dragon doesn't have a tail...
"An octopus gone mad and violent who has settled in the city sewers. It is different from other, normal octopi; through an underwater science experiment, its body's balance has been completely upset, and it has grown to a humongous size. Marked by its poisonous-looking color. Also has a hard, sharply-pointed head with which it can also pierce enemies. Normally, it entwines its enemies in its tentacles with its giant suckers, squeezing them to death."
Underwater science experiment? Sounds a little bit like something Dr. Lugae-ish...
"Assault Door
A living-door beast. At once a trap and a monster. Once one lets one's guard down and, thinking it an ordinary door, tries to enter, the door suddenly slams firmly shut, and its victim is thrown into another dimension. Then, the victim is absorbed right down to his very soul. They [the Assault Doors] lie in wait in places where people are likely to pass by, such as mansions or residences. Will not stop moving ahead until defeated."
Trap Doors that may have at one time been used for more than just the Sealed Cave? That could have been neat.
Well that's all I could reason. That was an interesting little journey to see what was left out.