I think the difficulty in increasing the number of II properties on the World Passport Collection list is that the six DVC resorts are so much better than the average timeshare resort.
Let's suppose a DVC family trades 263 points (Dream Season 2BR at OKW) for a 2BR during a popular season at another timeshare resort. If they get a 2BR at a high-quality, 5-star II property. such as a nice Marriott Vacation Club resort in agood location, they'll probably be pretty happy. If they get a 2BR at a second- or third-rate timeshare property, they'll probably feel cheated.
So DVC provides a limited list. And even with the current list, it's likely that DVC exchangers will react to many properties with, "Well, this place is okay, but it's no DVC."