There are a number of exceptional mods out there, and this particular gem is certainly one that stands out - Tale of Kingdoms! This massive medieval-themed mod includes a staggering amount of content, featuring interactive NPCs, quests, a guild leveling system, all leading players through a journey to start up their own kingdom! As you progress, you can choose a site for your new city, and have various specialist buildings constructed for your new-found kingdom's benefit! The kingdom can expand over time, and will begin giving the player-king their own stream of income, founding their own army and more! The mod does a remarkable job of integrating itself into normal Minecraft play, without preventing players from doing their own thing, or feeling out of place.
Author TyberAlyx notes that the mod is currently still in development, but is playable right now, and expanding all the time. Give it a look!