I know Notch mentioned making a 'hard core' mode where your world would get deleted upon death.
Well drawing from that idea.
(I am uncertain wether the game keeps track of player placed blocks or not I am also no artist or code capable so bear with me)
Upon death in HC mode all blocks placed with a player's hands would be put into a pseudo save file for the next HC map created. The blocks would be 'stickied' to each other in a group IE a house built would save as a package of player placed blocks. Chests would keep items inside of them. Trees/saplings/dirt dont count.
When a new HC map is created, new chunks created would have a chance (1/10 or 1/100?) of spawning with one (1) of the grouped packages. (grouped package would spawn at the same elevation as it was created, so not to have a cottage originally on a hill appear underground) The group package would have randomly destroyed blocks (1/5 chance per block?) then that particular grouped package would be removed from the pseudo save and be part of the new world. If a group has a chest associated with it. a spawner will be placed with it. perhaps one with each chest in the group.
This would present ruins that the player could encounter. And a feeling of nostalgia for the player as they encounter things they created in previous lives albiet with some wear with age.
This 'pseudo save' folder would keep going.
IE: build house in life1. die house gets saved.
life2 get creeper bombed before house group could spawn.
life3 find ruins of old house and be like "man i remember this, so many memories, i love this old house.
im gonna fix it up" Get murdered from a zombie inside the chest room, cry. start life4.
Perhaps there could be random generic ruins that could also spawn.
groups spawned as ruins don't count as "player placed" blocks so they will not be saved. (if this wasn't obvious)
No clue how hard this would be to code, if it were even feasible. I would say this should only work in HC mode, but i suppose it could work with other saves as well. perhaps have an option button that says 'ruins off/on' to turn this feature off or on. when off MC would not save nor load ruins.
I love the idea of wandering and finding the ruins of an old civilization, how much cooler would it be if the ruins were your own.