Not a 'tree' house as such, but I do have a multistorey cliffside build outside in Bouldertree Refuge (not sure if that counts ) I'm on PC/EU, you're welcome to visit any time, send me a message if you are too.
For some good structural materials, I recommend using: * Elsweyr Platform, Wooden Small * Elsweyr Wall, Rough Wooden * Elsweyr Pillar, Rough Wooden (if you need some cheap support structure) * Argonian Lattice, Rough * Murkmire Walkway, Reed * Murkmire Platform, Reed * Murkmire Ramp (Marshwood and Reed) * Orcish Platforms * Limestone Stairway, Natural * Coral Shelf and Limestone Shelf (if you want an organic-looking stone platform) * Rough Planks (old cheap sructural planks but they're a bit long and wonky) * Rough Block, Stone Slab * Rough Platform, Stage * Wood Elf Union Trellis (really nice natural latticework for a wall or a ramp).
Off the top of my head these are usually useful, and have natural vibes. You can check out many images of Structural furniture that you may like here: https://eso.mmo-fashion.com/category/database/item-type/furnishing/structures/
And here are some quick shots of my Argonian platforms - although it's also not a tree house, just a build on top of Ald Velothi. But maybe it will give you some inspirations for structural items.