Imagine Mount and Blade 2, but set in Medieval Europe between the 14th and 15th centuries. Now mix in Crusader Kings and Total War and you're on the right track.
You start off as a Lord for one of the many nations of Europe of the time. You need to raise armies when required and fight for your monarch and also run your land, but with enough power you can make a grab for the throne. You can then run your kingdom and conquer other nations, although they'll be a range of victory conditions.
Like Mount and Blade and CK, they'll be deep RPG elements. If your character dies then you play as whoever next in line is. There will also be deep diplomatic options, so war could be the last resort.
The map would be the exact same size as Europe, so just over 10 million kilometres sq, and you can explore every part of it. Every major city, town known village and natural site will be present. All cities will be exact recreations of how they looked in that period. If you wanted to ride from North England to Southern Italy then it would take you months in real time. Fast travel would be an option, but your character would age appropriately.
In fact, every you do in the game would take real time. Eating would take time like in real life. Drinking and hosting feasts would take hours. Sex would take as long as your character could keep it up. Your character ages in real time. Children grow up in real time. Pregnancy will take a real world 9 months.
Battles would be huge. Up to tens of thousands of NPCs mixing it up on the field of battle, and like real life they could take hours to resolve.
Basically, a game that you could be playing for decades.
Tell me this isn't the greatest idea of all time.