Pretty much just gonna use this post to write down my thoughts, because I have a lot of them and don't really know how to organize them.
-Breath of the Wild is a timeline Unification in which all three timelines have led to the same outcome, therefore you can have coexisting elements from each game.
-In the Child Timeline, we see the progression of events from Majora's Mask all the way through Twilight Princess and Four Swords Adventure. These series of events set the timeline up as a clean slate for further development and a possible unification opportunity is created. ("After penetrating Ganon, the four Links firmly seal him inside the Four Sword, bringing chaos and horror to an end. Peace returns to Hyrule, the souls of the knights are freed from their torment, and the people celebrate as all traces of evil that plagued Hyrule are vanquished. With Ganon defeated and sealed inside, Link returns the Four Sword that sealed darkness away back to its resting pedestal and the four Links become one again. After the battle, The Maidens of the Shrine use their powers and create a barrier, which is strong enough for the Four Sword to rest before the six maidens leave. Thus, Link's journey has ended and both he and Zelda leave the Sanctuary as Darkness fades away and Light endures in the Land of Hyrule, bringing peace and prosperity." -Zelda Wiki) connections to this can be seen in quotes by Zelda in Breath of the Wild, when she directly references the Child Timeline.
-The Adult Timeline follows The Hero of Winds and his journeys throughout the Great Sea and new Hyrule, and although it is NOT possible that the Hyrule from Breath of the Wild and the New Hyrule are one in the same there are quite a few connections which create a possibility that the timeline was unified in Breath of the Wild.
1) Breath of the Wild uses the Windwaker equivalent of the symbols of the three Golden Goddesses. Allow me to clarify. Sometime in between Ocarina of Time and The Windwaker, The Goddesses created their own respective pearls after they flood Hyrule and create the Great Sea. The purpose of this is to allow the destined Hero to find the pearls in order to open the Tower of the Gods and test his strength. Breath of the Wild adapts these and they are featured at multiple locations throughout the game. In Zora's Domain (BotW) Zora Royalty are featured as wearing the symbol of Nayru's Pearl. This symbol is clearly different then the one represented in the Spiritual Stones from Ocarina of Time.
1a) The Symbol of Din's Pearl is featured in the Ruins of the Temple of Time (BotW).
2) Koroks, Zora, and Rito are represented in the same space. Although it is true that the Rito evolved from the Zora, a timeline Unification would make their coexistence possible. A select remaining Zora resurge and prosper after the defeat of Ganon in Four Swords Adventures, which leads to their coexistence with the now flourishing Rito tribe. The Korok Population number DOUBLES, as it is possible there might have been a few remaining Kokiri Children from the Child Timeline who would assume the form of Koroks after the Unification of the Timeline. This could be why we see SO MANY KOROKS in Breath of the Wild.
3) Also (this is kind of irrelevant) but the Link's Champion's Tunic and Windwaker Link's Island Shirt are similar colors and feature the same type of design.
-Also just to clarify: The Great Deku Tree is not a timeline exclusive element, it can literally replant itself so even when it dies in Ocarina of Time there is still a replacement represented in The Windwaker.
-Okay so disclaimer I don't have that much evidence for the Fallen Hero Timeline so for now I'm going exlude it from this post but if you have any evidence from those games to support please post it!
The General Consensus is that each of the Link's from each of the respective timelines have thus far defeated their evil Motif and therefore prosperity (as far as we know it) is prevalent in each of the timelines present time period. Therefore the events of Breath of the Wild can take place as a unification of those timelines because there are no exclusive events that pertain to a specific villain. This is just the evidence I have found but if you guys have anything please post it! Or even if you see a hole in my theory let me know so I can work on a way to patch it and reshape the theory!
Until next time!
-Kolle