A replica of the Earth basically just means you don't need a map (assuming you've got a basic knowledge of the globe) and things are distributed in annoying ways that don't make for enjoyable gameplay compared to a generated map.
I would like to combine Minecraft with Mount & Blade, so we could all have our own nations and raise armies and have big huge battles, but we have to find/raise horses first and craft our own weapons and all the other Minecrafty things.
It would be exactly like Minecraft is now, but with these additions:
* A "forge", which is a larger crafting bench (grid of 5x5 or 8x8 or something).
* This would allow a greater variety of weapon patterns, so not just sword, but dagger,longsword,lance,battle axe,etc too.
* A "horse" animal, starts wild but tameable (like wolves) and then given a saddle to let them be ridden. There'd be different types (like sheep), and different roles they could have (e.g. packhorse,warhorse,etc).
* Land can be claimed by a player placing border flags, which have a radius they apply to (similar to torches) and if they form a circle the inside of that circle is also claimed (unless it contains other player's flags).
* Once a player has a certain amount of land, a new "animal" type of "peasant" is generated randomly, and if given weaponry they turn into a "soldier" that will fight for you and your allies.
etc.
It'd be completely unlike Civ - there'd be no tech tree, no city centres, no taxes, no special effect buildings.
Wonders would be whatever you wanted, and be a simple matter of respect/coolness, showing that you're so well defended that you've been able to gather resources and build this big cool thing.
Expanding your empire involves removing enemy flags and/or placing your own flags, but of course with enemy soldiers patrolling borders that would involve battles first - unless you mine your way past their defences - but you'd have to make sure to avoid the traps they're certain to set (like a hidden moat of lava, or a "trip wire" of redstone that, once broken, sets off alarms, etc).
It'd be far more unpredictable, because everyone can build their own bespoke systems of defence.
