Lava, some dirt, a grass block, and an oak sapling. Add 10 obsidian blocks and a piece of flint for a serious expansion of your possibilities.
With the sapling you get infinite wood. You can get iron from zombie drops. You can bonemeal grass (spread onto dirt) to get seeds for wheat. Spider drops - string - will enable wool and a bed. You can also get carrots&potatoes from the zombies. With wood you can build away from your dirt (converted to grass by now) and let passive mobs spawn (providing there are none in the spawn chunks). With iron you can get a cauldron which will fill with water during the rain. With water and lava you can create cobble and smooth stone. These allow building a furnace, smelting charcoal and making torches. With two water sources you can have infinite water, create a fish&drowned farm and get gold from drowned. With gold and an apple from the tree you can craft a golden apple. Using a witch, a zombie villager and the golden apple you can get a normal villager; with two of them and carrots/potatoes you can get a villager breeder going and get lots of valuable resources through trading - including diamond gear and tools. Mushrooms from wandering trader for mushroom stew and suspicious stews done with flowers. Fishing will provide a number of enchanted books, you can also obtain bamboo in a jungle biome. You can also afford some automation with redstone dropped by witches.
If you started with obsidian, you can go to the Nether. Infinite bounty of lava, nether quartz needed for more advanced automation, blaze rods for brewing, a shortcut to finding mushrooms. A pigmen farm will earn much more gold than a drowned farm. If your sky world has an End portal, you can make eyes of ender and go to the End, fight the Ender Dragon, get infinite Ability to get
If your world has the end portal, you'll also be able to get shulkerboxes and elytra, never mind all the random loot (including normal diamonds, allowing you to build enchanting table). If you got sugarcane from a wandering trader, you'll be able to make rockets to power your elytra too.