I've done multiple builds: most on peaceful as im a wuss.
Underground cities consisting of massive caverns blown out by ludicrous amounts of tnt. Currently unfinished, planned rail way system between large number of chambers and areas, banks and such.
A rail line that connects multiple 'countries' as denoted by flags made using coloured letters. Currently countries and other such places (made and planned) include:
Made: Germany (First zone, contains main house, entrance to the mine), the Hub (Rail hub leading to other places, planned expanantion into a 'rest shelter' with a large observation deck), Russia (Main lodge, build on islands, flag build on large natural pillar), Canada (Largest country, having 3 lodges currently. Planned expantions include: more houses, open bonfires, open crafting areas, raised walkways, as well as connecting it to the rail line.), Switzerland (one lodge, with kitchen area and lounge area. Large wooden walk way leads to to the top of the mountains its in.).
Planned: The Wastes (Large empty area between Hub and...), Creeper Land (A land blown apart by tnt. 'nuff said.), Atlantis (Entrance already slightly dug out beneath the ice in Russia.)
A meager tree house from what ive seen, many, many patches back
A large factory which included many different rail lines including rail lines for: lava/obsidian farming, underground quarries, underground crew quarters, 'secret' portal to hell with a portal inside hell leading into a massive treasury.
Like I actually pay attention to lectures.