Upon entering a new world the game will ask you “Do you want to say anything?”. After you’ve got the advancement «Free the End» or killing the dragon you’ll receive a written book with a text in it. I’ll put the screenshot and explain each element:
Tier # means how many worlds you've conquered.
#% increased item quantity/drop chance will be increasing with each tier. The percentage can go beyond 100%*. Can't be re-rolled.
Modifiers are randomly picked. One mod will be added with each tier. You can re-roll mods on anvil with nether stars. Player can't get potion buffs, only non-damaging debuffs (they last forever and can’t be removed with milk) and life quality mods (e.g. auto-smelt).
The book will contain a seed for these mods at the last page. For example seed 5_32_27_AS_HA_S1_SR20. AS - auto-smelt, HA - hostile animals, S1 - speed 1 for mobs, SR20 - increased spawn rate, 32_27 - item drop and quantity, 5 - tier. If it’s not realistic then how about to item drop and quantity will scale with the amount of modifiers and modifiers will have only one symbol for each. You have to type the seed when you enter a new world.
The written book can be dropped with the revived dragon if you lost one.
*That means if you have 120% increased item quantity then you have guaranteed +1 to the dropped item and 20% to get another +1. Drop chance works like that: if you have 10% increased drop chance, ender pearl drop chance will be 55%, not 60%. If you have 110% then you will have a 10% chance to get random lvl 1 enchantment on the tool or weapon, or get upgraded version of an item (e.g. eye of ender instead of ender pearl).
P.S. My idea isn't unique. Just felt that minecraft lackling of endgame content. I want to neither explore nor build. I want to beat the ender dragon over and over again, but with some spice.