What if you turn the game planet inside out? The player is inside a huge bubble, where all the attributes of the earth (mountains, forests, seas) are the same, only they are not outside the sphere, but inside. The landscape does not disappear over the horizon, but rises, so that in clear weather you can see the surface on the other side of the world, for example, from Australia you can see Britain just with binoculars.
That’s feasible, but you need to customize world gravity and physics. Depending on how big the diameter of your planet would be, it would lead to some interesting gameplay.