To imagine the question lets have the following scenario:
We have Terraria (a 2D Platformer revolving around harvesting resources and building stuff). We have a large 3D sphere populated with resources (static) and creatures (dynamic and reactive). If the world of Terraria was 3D instead of linear 2D (with 3 layers in the 3rd dimension in total), what are the different ways one could deliver the visualization of such a world/game?
The example I have given above is for a 2D Platformer based on a 3D world. For the above case - the question is easily solved somewhat by converting the 2D platformer into a top-down planar game (Civ V to a limited extent - again represents a cylinder not a sphere; Rimworld nails the top-down solution almost perfectly, but some rasterization using tiles was required) - but lets avoid that and explore some better options.
Another example I would like to give for a top-down 2D planar game based in a 3D space:
We have a classic Top-Down space invader shooter, in this case, space is completely flat and planar - with limited depth in the 3rd dimension. What if the depth was infinite in the 3rd Dimension? What if the dimensionality of battles in realistic 3D space games were more accurately simulated instead (attacks arriving from multiple dimensions - and not just the same plane)? Ofcourse, keeping the 2d concept of the visualization itself in mind. What are the different ways to make this possible?
And also, is it even practical?
This is not easily solved by turning the UI into a platformer. Homeworld tackles this only to a certain extent - but it still limits the depth and reduces the significance of the 3rd Dimension.