Guenivere was intended to be playable and act as a second lord.
Tate and Lilina may have been related.
Zephiel and Murdock may have been brothers.
Zephiel's weapon, the Exxacus, had durability and was intended to be acquired after defeating him. In addition, the eponymous weapon was also intended to be acquired the same way.
Yahn survived his battle and would be fought alongside Idoun as a Dual Boss.
Data exists that suggests that Blizzard might have been intended to have been a long-ranged weaponry. Evidently, Meteor was also supposed to make a return but was not coded into the game.
There may have been Dark Dragons fought alongside Idun. Complete with droppable Dragonstones that Fae would have been able to equip. The jury's still out on if they were supposed to be mass-produced versions of the stronger version of the Earth Dragons seen in the Archanea games, however.
Fire Emblem 7:
Durandal had a different animation and appearance, and was intended to be used by Lyn, too.
Vaida was originally supposed to be a Bernese Noble, and would have been revealed as Heath's sister.
Raven originally had grey hair, and Heath had red hair.
The Dancer and Bard rings originally had ribbon icons and could be used as a menu item.
The still images were in full colour and did not have a Sepia-hue over them.