Way back in the MS Flight Sim days they had "cloud impostors" which is exactly what it sounds like, just like a normal impostor a 2d pre-computed plane of what the volumetric cloud looked like from afar.
Since with the above (no less than 3 excellent papers) you're marching through a volume you're then concerned with volume marching samples, and not at all with LOD. The trick would be some way to skip as much space as possible and only march once you actually get to the "cloud" layer you want. Because it's per pixel otherwise cost would then be a relative constant.
On the fifth day of creation, to hide the z-fighting and to prevent distance objects from suddenly popping into view, God added curvature to the earth.
Atmospheric fog and curvature together would probably help set an absolute limit to your terrain. Any ridiculously high objects - like mega towers or mega mountains, you could add just those (hopefully few) extreme situations to your skybox.
Yeah, on earth, the horizon is about 5km away, or 36km if you're on top of a 100m tower. 5km is definately feasible for most games. 36km is feasible if you've got great world LOD'ing
You can also just stop rendering things past a certain distance as an extreme/simple form of LOD. In my game, the world only exists out to about 5km, and suddenly stops. The atmospheric scattering effect draws a curved horizon in the distance, with fog thick enough that you can't actually see where the ground ends and the edge of the world begins -- it just looks like a smooth horizon!
If you use the debug camera to fly 200km straight up into the air, the horizon gets more and more curved until you're in space looking down at a pale blue dot like a space simulator Even though it's just fog (you can't see any continents or oceans), it's a pretty amazing effect to make it feel like the game world is boundless.
Haha! That's crazy, I want this game you've been working on to come out now just so I can see what this looks like