For a long time I wanted to make entire planets the player can navigate, with other seamless environments like a ring world similar to the first Halo. Merely getting sphere shaped terrain is easy, but everything else makes this not worth it: You need to use "spider physics" to have gravity work not in one axis (-Y) but attract or repel from a world position, this gets very complicated when it comes to properly rotating characters and having pathfinding work too... it's otherwise harder to align stuff accordingly compared to everything being in flat space.
There is a better way though... at least in principle and for other engines, I don't know if this is even theoretically possible in Godot. You can use a vertex shader to distort everything inside a cubic area so that it maps to a cylinder or sphere: This doesn't change the position of objects or even vertices internally, but displaces them on the GPU so from any distance and position you see a normally flat world warped into a sphere / ring / plateau without being able to tell a difference.
The ideal implementation would be something like this: Everything warps around the world center of '0 0 0', the shader takes a scale as the parameter so for example 1000 if you intend your world to be 1000x1000x1000 units. Any point at position X: -1000 will seamlessly connect with everything at position X: +1000... same for other axes if the system can allow it in multiple directions, a flat plane can't be warped into a sphere without cutting or shrinking / expanding some pieces so I doubt it could do a full planet. Obviously the game must separately ensure any entity going below -1000 or beyond +1000 is teleported across the seam without seeing a difference, or that the heightmap terrain is identical across the seam.
Obviously there are a lot of complications that may make this impossible. For instance something that's behind you in flat space might appear in front of you when distorted by the shader, view frustum culling may need to be either disabled unless it can somehow run after the distortion. There's also the issue of things other than geometry such as lights or particles: It's not just every vertice but also every particle that must be moved to the correct location, light sources need to still shine in the correct spot and have their origin offset... sound would be so complicated I'm not even adding that to the list, audio can still work in flat space without much accuracy loss.