OK, let's go by the list
1- Multiplayer is possible, I'd say this is pretty independent from other engine chores - generating and rendering the world. The generation is random but deterministic, meaning the same stuff is generated at the same place every time. Sure, there are complications resulting from the huge world and how it all will be managed, but we have some ideas of how it should work.
2- Destructible terrain is possible; dirt roads can be seen as an example of it too. Of course heavier modifications aren't a problem as well. We have been thinking also about having destructible trees, this may be possible too once all other more significant issues are resolved.
3- There are other types of roads already but of course more road types can be added too. It involves defining a road profile, a dirt overlay texture for tinting the road surface (combined with parametrized fractal colorization) and defining what materials constitute the road layers (border/pavement/markings). Types of markings can be defined separately. Other parameters can be specified when physically placing the roads, per road segment - width, border, slant, elevation above the terrain and current marking.
4- Well there should be at least tunnels. Caves would need a generator. Anyway, it's doable.
5- We are more inclined towards making real planets now, but to allow for different settings, "realms". So we could have Earth in a prehistoric state, a current one and some futuristic and apocalyptic one. It would all share the common basis (elevation and climate data) but will use its own textures, vegetation and distribution rules and overlay it atop the world.
As for other planets, we can do Mars and Moon and other rocky planets for which there are some data. Of course it is possible to create a completely artificial planet too, if one provides its elevation and climate data and compiles it to a dataset usable by the engine.
But the priority is to make Earth as it had looked some thousands years before, without effects of civilization, and to have vector data layer where these effects will be recreated. Probabilistic fractal engine can then show the world slowly decaying back to the natural state too.
