Remains

After an old world is deleted, the builds the player constructed in that world should have a chance of spawning in their new worlds as remains, these remains could be like run down versions of bases from your old worlds, there could be cob webs all over them, and you could have a chance of finding your old chests still with the old items in them. Maybe if you had pets in your old world little grave sets could form near any old bases you find, etc.