At least from working for a major company, I know that the absolute biggest trend right now is data collection and using it to drive repeat customers. That doesn't seem like it'd be an issue for Disney, but it also wouldn't be an issue for the field I'm in, either. I work in beer for one of the big brands, so just based on what I'm seeing, some predictions for this "membership":
If they roll with a membership, so to speak, I can see them obviously including tiers. Either you pay for the tiers (think annual pass tiers), or you have a base payment or a base/then platinum price where you basically earn points toward stays, discounts on dining, discounts on your next stay, discounts on merch and all. Maybe you can even unlock room upgrades or FP+ selections.
If this happens, I think bounceback offers will end—instead, you'll earn points toward discounts on your next stay.
I think dining plan won't come back, since it'll complicate things even more. Instead, you can earn "free" meals or some sort of discount on food at restaurants.
I know, there's always going to be a demand for Disney, but Disney (and every other company) is sinking tons of money into data collection. They did it with the MagicBand and the MyMagic+. Doing a membership program will help them track how you spend, and by giving rewards for certain purchases, they can see what motivates each and every customer—so they can tailor trips and future offers just to you—which will in turn drive more people to purchase.