In regards to how the final battle was originally going to be like.

[Just call me AL]

I've been thinking about how there was an unused item in the game, specifically, the Demonstone, and how it could have affected the final product.

For anyone unsure of what I'm saying, the Demonstone is indeed an unused item. Its weapon data is identical to the "Dark Breath" weapon Idunn uses at the end of the game, indicating that it was meant to be used by her. However, the Demonstone also grants its user boosts to both Atk and its defensive parameters, which isn't unusual for Dragonstones in the series.

If Idunn was supposed to have the same stats she has in the final release on top of the Demonstone, I dare say that it might have led to a similar situation to Ashnard, where only certain units would have been able to defeat her, including your Lord and your Manakete unit. Crunching the numbers, and I could be wrong, Druids could go toe to toe with her as well, under this assumption, but would require max magic and Apocalypse.

Then I'm remembering that there was some hearsay in regards to the prototype. Apparently, Idunn was supposed to be accompanied by "Dark Dragons," with droppable Dragonstones, in the final battle. I'm also remembering that "Dark Dragon," in regards to Binding Blade, is a mistranslation of the kind of Dragon that Idunn is for the whole game, Demon Dragon. What I'm wondering is that, if true, were these Dragonstones supposed to be the same kind of Dragonstone originally intended for use by Idunn? And if so, considering the fact that the Demon Dragon has the unique ability to create War Dragons, could these "Dark Dragons" speculated to have accompanied Idunn in the final battle have been literal copies of Idunn herself, made by her? Kinda terrifying to think about, to be honest.

[Mukmuk]

Well, probably the #1 reason why they didn't add the Demonstone is because (actually, multiple): 

1) The Demonstone is busted. Nobody can damage her, even with legendary weapons. Lilina at Max Magic can do like 2x2 to her. Even Apocalypse can't do enough to her. It'll do, like, 6 damage? Then your poor Druid will perish. 

2) The Binding Blade can break. Thus meaning that Idunn is an impossible boss to defeat (this was mentioned by Mangs). 

So, I can see why they didn't choose to do this. At least they could have tried to make the Binding Blade a little less broken and increased Idunn's stats or something... 

[Acacia Sgt]

Regarding reason two, they could've just made the Binding Blade unbreakable, like the Falchion, if there really was a concern that the player would used it all up before reaching Idunn.

Actually, it sounds like it would've been just like original!Medeus. Immune to ranged attacks and magic; and high enough defense only Falchion, Mercurius, Gradivus, Divine Stone, and Pachyderm could hurt him. Again, if the Binding Blade breaking was a concern, they could've just made it unbreakable, like Falchion.

Considering reaching Idunn already requires special requirements, it wouldn't have been that much more demanding, I'd say.