Each phase leaves a game you could hand to someone. Built zone-by-zone instead, there would be nothing playable until the very end — which is how most projects this size quietly die.
The capital. Three outdoor maps and four interiors — bank, inn, vendor, three class trainers, a respawn point. No combat inside the walls.
Five outdoor maps plus the cellar. Four mob types, three gathering nodes, four quests, one named boar, and the tutorial that runs on first login.
Experience curve, every variable, both crafting professions and the reputation bands — set before a second zone exists, so everything after is content work rather than systems work.
Five outdoor maps and a two-floor open dungeon. The real test of whether the combat model is fun in a corridor.
Five maps plus the longhouse, the first outpost, and the Fenmother's timer as a rehearsal for the world boss.
Two surface maps, three open cave maps, three instanced. The first party dungeon.
Five maps plus the barracks. Elite mobs, the reputation vendor, and the payoff to the Thistlewood bandit thread.
Three outdoor maps, two instanced, one world boss on a three-hour timer. The final chapter, and the hook for whatever comes after.
If this turns out to be more than one person can carry, the cut is the Ashen Steps: end at level 34 and move the Riven Stone forward. Six well-populated zones is a game. Seven half-built ones is a project people stop logging into.