I’ve always been quite fond of placing Magical Portals leading to other Places and Planes in my games. During the Higher Levels of Play (usually post-Domain Tier) Planar Adventuring can be very enjoyable and sometimes necessary to accomplish certain things. It often reignites some of the excitement of the Dungeon Tier: Travel is often hampered or restricted in some way and those precious Resources begin to matter once more. The Environment might be more Hostile or inimical to life from the Prime. Wonder and Danger can lurk around every corner/with every jaunt.
But these Planes need not all be such monolithic and grand places to peregrinate to. I’m very fond of the highly specific, smaller places (some might be only every be explored briefly or may only be a few “Hexes” across in terms of scope) often sought out to achieve very specific goals or tasks, or inadvertently stumbled upon on the way to somewhere more substantial or through misadventure.
Here are a Hundred Ideas for these smaller Planes to sprinkle within your cosmological framework. These could be destinations for those Mysterious Magical Gates, or even only possible to visit in less conventional ways. Some even beg me to be flesh them out further into more fully-fledged Troika-like Spheres! It can be quite perplexing and mysterious how these Places end up working in terms of their wider implications, but I tend not to break my brain over such things and leave that as an exercise for the Players to unravel 😊.