I recently discovered a Minecraft map where somebody had taken the entire Earth and shrunk it down to 1:1500 scale, at which point a number of relatively small features (Cape Cod, the Aegean) become scarily visible.
It would make an awesome Civcraft-type server, I think. Take the monorail from New York to Los Angeles, or have an underwater train track running from Cape Town to Perth?
The way I'd do it is that when you log on, you get put at your "real" coordinates (ignoring the how for now). Build your home town or just bury it in TNT!
Note that at 1.5 km per block, most towns would be difficult to fit onto the map. I think the best approach might be more like 10:1 to 100:1 scale, fine enough to allow voxelized versions of landmarks to fit while minimizing the problem of having way, way too much spaaace on the map for the number of players.
We would still need to enhance Nether transport so it's a further 20+ times more compressed. On a 100:1 map, you could sprint from New York to San Fransisco through a x20 Nether in 25 minutes.
That'd be, what, 225 times the size of a 1500:1 map? How big can servers get? I mean, obviously a vanilla map can generate new chunks pretty much as far as it needs to, but we're talking about creating the whole map before anyone sets foot on it...
EDIT: OK, I pulled out Wolfram Alpha and got some math done. The circumference of the Earth is 40075 kilometers, which, at a 100:1 scale, means 400.75 kilometers east to west by 200.375 kilometers north to south- which means 400,750 blocks by 200,375 blocks.
The largest Minecraft world currently under construction is called
Aerna (NSFW due to an obnoxious sidebar advert), and its area is about 5.24 billion blocks. A 1:100 Earth server would measure about 80 billion blocks, so we're talking something well into an order of magnitude larger than the largest Minecraft world in existence.
Now, if you shrink this down, you can still get a really large and buildable server- a 1:300 map would be about 9 billion blocks in area, which is within striking distance of Aerna.
The real question is: what can you build on a map of that size? I downloaded the 1:1500 map and flew around it in Creative mode for a while. There are a lot of quite small features that are recognizable but too small to build much on. Cape Cod, for example, is a peninsular a block thick jutting out into the Atlantic, and although you could build a New York, a Minecraft town of serious size centered on New York would probably stretch from Maine to Baltimore and west to Buffalo.
On the other hand, regions like Antarctica and the Sahara will be no less habitable than any other part of the map, so the big cities might develop there. (But of course there's a certain emotional attachment people would have to building Chicago or London or Cape Town, and those will be prime real estate for that reason.)
As for a 1:10 Earth...that's 8 trillion blocks of area. Are there any servers that could run something like that that don't belong to the Pentagon or the NSA?!