Just want to say, here are the lists of memberships that exist or have existed in other resorts that aren't an annual pass:
1. Hong Kong's Magic Access Program - basically an annual pass except you can also buy 2 year passes
2. Shanghai's seasonal (3 month) and 1/2 year passes
3. WDW's pass for the Mickey's Not so Scary
4. Funderful Disney for Tokyo that gives discounted tickets, special perks, free gifts, etc. - This is more like a super upgraded D23 that's park oriented
Here's the groups of customers I think the annual pass appeals to that Disney will try to separate:
1. People who go frequently but primarily just care about access to their favorite rides, shows or other standard park experiences on a regular basis. Such people may use the park as a place to hang out and this is the group that Disney wants to limit if they don't also fall into one of the other categories.
2. People who will visit the park for special events and activities. Think Food & Wine, new ride openings,
Disneyland's Anniversary, etc. Since this group generates revenue by buying seasonal food/merch, it'll be a problem if Disney didn't allow them access on important days or blocked them out for entire seasons.
3. People who won't be willing to pay ticket prices but are willing to visit a few extra times a year to make an Annual Pass worth it such as those from out of town. Disney wants to keep an annual pass around for these people and wants a reasonable reservation system that they can work with.
4. People who buy a pass and forget about it or only do it for the bragging rights, sometimes forcing themselves to visit a few extra times to make the pass "worth it". Disney likes these people too.
5. Frequent spenders such as those who go to the parks to specifically have Disney themed food or regularly buy limited edition pins. A major risk of making the new membership program too restrictive is losing this profitable group of people.
The way I see it, a flex pass system seems like it would make the most sense for Disney because a shorter pass could price out a lot of the groups that make money. If they wanted to be fancy, they could also create a points system like what DVC has but I think that's unlikely.
An interesting thing to note is that the USD is falling against foreign currencies so there may be a big opportunity to shift focus to foreign guests in group 3 and especially Japanese guests who fall into group 2. Japanese spend way more on food and merchandise than the average US guest, are far more likely to stay at Disney hotels, and already make up a sizable portion of Hong Kong memberships and Aulani DVC.