Sounds a bit like Lorwyn-Shadowmoor.As a campaign element that I was considering a while ago, I had the shadowfell and feywild as the same place, with only one existing at once. Essentially this single plane "orbited" around the material plane going from light to dark to light again. This happened over centuries so tales of malicious fey tended to be thought of as just tales but at the start of the campaign, the Feywild was transitioning to the shadowfel again so those tales were about to become much more real.
It's been a while since I thought of it, but that could very well be where I got the idea.Sounds a bit like Lorwyn-Shadowmoor.![]()
Your comment here has reminded me how I had the feywild/shadowfel transition, it was to do with it's relation to the positive and negative planes. The plane itself orbited around the prime, as it rose (and approached the positive energy plane/font of life) it transitioned into the Feywild. As it fell (and approached the negative energy plane/Entropy/The end of all life) it transitioned into the shadowfel. I recall having a planar set up where in the celestial planes above energy flowed outwards providing life to all the multiverse, celestials being the closest to this life giving energy. The energy then flowed down through the planes, passing through the elemental and prime planes (essentially on the same level but with the prime nested within the elemental planes) and then downwards to entropy. The lower planes were closest to entropy and the fiends there could draw upon its power.As the OP states, the Shadowfell and Feywild are echoes/reflections of the Material Plane. The Feywild is closely tied to the Positive Energy Plane, and the Shadowfell is closely tied to the Negative Energy Plane.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.