This mod adds 4 chest items that can be opened to aquire items, defined by a loot table.

By default these chests can't be aquired anywhere in game other than the creative menu, so you will need a mod to either add them to mob drops, random chest loot or quest rewards.

To add your own loot, just override the loot table "lootchests:serious_loot".

It has 4 tables entries, "common", "uncommon", "rare" and "mythic".

To learn more about how vanilla loot tables work, look here: https://minecraft.gamepedia.com/Loot_table

Either add your own loot table file to your world save folder, or use mods that modify it. Personally i recommend using LootTweaker.

Vanilla Example:

{
  "pools": [
    {
      "name": "common",
      "rolls": {
        "min": 1,
        "max": 3
      },
      "entries": [
        {
          "type": "item",
          "name": "minecraft:stone",
          "weight": 1
        }
      ]
    },
    {
      "name": "uncommon",
      "rolls": {
        "min": 1,
        "max": 3
      },
      "entries": [
        {
          "type": "item",
          "name": "minecraft:apple",
          "weight": 1
        }
      ]
    },
    {
      "name": "rare",
      "rolls": {
        "min": 1,
        "max": 3
      },
      "entries": [
        {
          "type": "item",
          "name": "minecraft:diamond",
          "weight": 1
        }
      ]
    },
    {
      "name": "mythic",
      "rolls": {
        "min": 1,
        "max": 3
      },
      "entries": [
        {
          "type": "item",
          "name": "minecraft:nether_star",
          "weight": 1
        }
      ]
    }
  ]
}

LootTweaker Example:

import loottweaker.vanilla.loot.LootTables;
import loottweaker.vanilla.loot.LootTable;
import loottweaker.vanilla.loot.LootPool;
import loottweaker.vanilla.loot.Conditions;
import loottweaker.vanilla.loot.Functions;

val table = LootTables.getTable("lootchests:serious_loot");

// Grab all the existing pools
val common = table.getPool("common"); 
val uncommon = table.getPool("uncommon");
val rare = table.getPool("rare");
val mythic = table.getPool("mythic");

// Set the rolls. By default, each pool has a minRoll of 1, maxRoll of 3
common.setRolls(3, 10);

// Add loot to pools
common.addItemEntryHelper(<minecraft:stone> * 16, 50, 1, [Functions.setCount(8, 16)], []);
uncommon.addItemEntryHelper(<minecraft:cooked_chicken> * 4, 50, 1, [], []);
rare.addItemEntryHelper(<minecraft:diamond> * 4, 20, 1, [], []);
mythic.addItemEntryHelper(<minecraft:nether_star>, 1, 1, [], []);

SeriousCreeper's Loot Chests