So I was thinking about the similarities between Morrowind and Shivering Isles (giant mushrooms, Hungers etc.) and while Morrowind isn't an extension of the Shivering Isles as it has numerous differences I couldn't shake the feeling that maybe some things we think are native to Tamriel actually come from various realms of Oblivion.

For example, we know Vampirism and Lycanthropy come from Molag Bal/Hircine respectively, so how much could daedric princes have influenced Tamriel? Netches look outlandish but then again so would Minotaurs to Dark Elves visiting Cyrodiil, could either of those (or any creatures we have encountered) be influenced by Daedra?

Or is it that the realms only look the way they do because the daedric princes are influenced by Nirn (were the first books written by mortals, or were they found in Hermaeus Mora's Apocrypha before that)? Or is this just a "What came first the chicken or the egg" type question and no one is too sure?

Also, does anyone know if any other Oblivion realms share similarities with Nirn?

The standard explanation given for why the flora and fauna of Morrowind (Vvardenfell in particular) being so alien compared to Cyrodiil and Skyrim is that the region is isolated by mountains and the climate changes caused by Red Mountain make it very hostile for the creatures that aren't native to the region. Think of it as Tamriel's version of Australia, only you get massive fungi and giant bugs instead of eucalyptus trees and kangaroos.

The daedra have certainly left their mark on Nirn though--Azura is attributed with making the dunmer dark-skinned, ogres and orcs have both been associated with Malacath, and at least one Khajiti myth attributes their existence as we know them to "Azurah" (who may or may not be Azura, depending on who you ask).

I have a feeling the giant mushrooms in the Shivering Isles were more an homage to Alice in Wonderland than a reference to Morrowind, but since the daedric princes can shape their realms to their liking, Sheogorath may have decided he liked those Telvanni mushrooms enough to put them in his realm.

As far as realms of Oblivion resembling Tamriel, Molag Bal's realm of Coldharbour is supposed to resemble a twisted version of Nirn. I can't remember any other references to Nirn in Oblivion offhand at the moment, but there is at least that one.