While definitely ambitious, a lot of the "basics" are already in place. Economy, inventory, travel, combat, player/NPC interactions, etc. As above, you start by organizing into groups of major gameplay systems, then iterate on the features, going into smaller and smaller details.

Take AI for example. Start with a "dumb AI" that spawns, walks until it hits something, turns and keeps walking. Kind of like a Roomba. Next pass, you add interactions. They might notice and look at you, or move out of your way. Keep adding details like more advanced interactions (e.g. playing video games, and using vending machines). This last part is actually what's being worked on now for AI.