I agree it's always fun to know these kind of small facts .

This is correct. Ocarina of Time was supposed to fully take place inside Ganon's Castle in the same way as Peach's Castle acts as a hub in Mario 64. Also, the fact that Ganon's Castle today contains six arms with different themes is evidence of that. Each arm was supposed to be a different 'world'.

Also in Ocarina of Time, the Forest and Water themes in the game were supposed to be Wind and Ice respectively. These themes made it very far into development, before they were changed into a more basic theme fitting with the others. Because of the late change, the Forest and Water wings of Ganon's Castle in fact still contain only Wind and Ice type elements, while the Forest Medallion contains the picture of a fan and the Water Medallion a picture of a snowflake!

Ocarina of Time was originally envisioned to being played in a first person perspective.

Haha, glad I managed to include at least one new piece of information for you. :D
I actually really like this thread idea though. Here's another little bit of trivia, though maybe a bit more well-known (?):
In Paper Mario: The Thousand Year Door, Junior Troopa from the first Paper Mario game makes a cameo of sorts. After the events on the express train, the player receives a letter from Zip Toad. In the background of said letter, you're able to see Junior Troopa flying around.

Personally, I loved this guy, so I was always kind of sad to see him not make a real return after the first Paper Mario game.