I’ve long liked the idea that the feywild and the shadowfell are so metaphysically “close” to the material plane that one can wander into them entirely by accident if one strays too far from the safety of settled lands, and the line between “the wilderness” and “the feywild/shadowfell” is very blurry. I also mention the two mirror planes in the same breath because I kind of see them as two sides of the same coin. There’s a lot of overlap in myth and folklore between the land of the hidden folk and the land of the dead. My personal preference is to make the shadowfell just be the feywild’s underdark, but I’m not sure if that quite fits the “fundamentally 5e” bill, so maybe just making it the unseelie aspect of the fae realm works better.