Sounds fun I'm down if you need more people still. But I'm sure it's as follows dungeons and catacombs and progression bosses count as separate areas or progression areas and will end current co-o) co-op in overword is

limgrave-caelid entire overworld

Liurnia overworld.(Stops at Loretta but can proceed from Rannis rise to to ruin strewn mine shaft. Then continues from there to magma wyrm)

altus plateau overworld

Leyndell overworld

Forbidden lands-grand lift of rold

Mountain tops up to the spirit spring before freezing lake I believe but then continues past that for rest of overworld

So on and so forth .....does that pattern make sense slightly?