I think I may do a combination of all these ideas. After Hoffy posted, I decided I would do a swamp. With that swamp comes two other maps. There is an underwater community for the Zoras and then they also have a home underneath the rocks in some caves similar to their abode in OoT. :] The water in the swamp is going to be green and unhealthy for the Zoras so you could consider that poisonous and a monster in the dungeon is a practical cause of that. :] So, as I previously said, their race is in danger. They've formed an alliance with the mermaids and asked them to give a mermaid tail to only that one person who they believe can save the Zoras and, in turn, once you attain the tail, the head mermaid asks you to help the Zoras who live south of them. And I could definitely put a small tribe of Zoras on a mini island looking for a new home. I already have to expand the map because the mermaids don't fit. Might at well throw another few islands on there. :] Thanks guys.
That sounds great!
But like Miles said, the poison was already done in MM and I think in Twilight.
Maybe the Lake isn't poisonous at all, just to dirty and mucky to swim in, maybe
it used to be a spring the Gerudos before they left for more land, which Link could
use an Item or a spell to purify the mucky waters thus turning it to a hot spring which
opens up 2 new entrenches to Lake Hylia and Mermaid's Lair.
Oh, and heres another, let's say the princess of the Mermaids was ill, dieing even, and they all thought it was a curse put upon them from the Zoras, which the main villain of the Story
tells them. And they believe because he also says he's the chosen one(showing them his triforce).
But Link proves them wrong by turning the swamp into a hot spring, and curing the princess with the Hot Spring water, thus proving he is the true Hero chosen by the Gods due to his "good deeds" and courage.
Link obtains this Item via the "Swamp's" Boss fight. And once you cure the princess, you get
the Item you needed from the start. Wow! That sounds easy huh...no, not really, the Zoras
have a even bigger part in this...right, Sprite Collector?