There's a reason why MMOs developed instanced content and tend to emphasize it - servers struggle as more players enter into open world zones. The overworld is going to end up limited by what the supported game clients are capable of handling.

The more you add to the overworld, the more the servers need to communicate with the player clients. That increases latency for everyone. The more reason players have for being in the overworld, the more players will show up and that increases latency even farther.

Right now, most PS4 players struggle during moderately sized hunt trains and large hunt trains are nearly unplayable for them. Even PC users with good systems but slower internet connectio speeds can struggle. Increasing the content in the overworld and thus the number of players appearing to do that content is going to make the situation worse for their supported game client.

How much more might be possible if SE dropped PS4 support and upped system requirements for PCs? It's hard to say but I definitely do not see PS4 support getting dropped until at least the 7.0 expansion.

More open world content might be nice but it may not be practical from a technical standpoint.