Zelda is known for it's epic, susspense filled, hardest boss in the game final boss fights. Having a secret superpowered boss would take away from this. (Oh and zelda isn't an action RPG, it has few to none RPG elements like it has no: -Random battles -Turn based battles -Leveling up -Ext.
-actually, there is an optional boss in ltp/four swords, namely palace of the four sword's shadow links, once you beat both games... apart from ltp/four swords however, i think optional mulltifloored dungeons like twilight princess' cave of ordeals might count as similar effect.. also, adventure of link had most of those rpg elements, and there are other games that had a couple of em as well even if not as many. personally i'd like to see more superbosses too, but anyways, just felt like i had to clarify. hope i didnt break any wiki-specific rules in doing so :I