I feel like the PWPs will exist in some form. For example, the police station, the 2nd floor of the museum, the train station remodeling... Those were PWPs too. I feel like aspects like those will still exist and obviously would make no sense to be furniture. They will probably be part of the crafting system (for things like remodeling buildings) or some sort of unlockable that you would have to complete some sort of goal to get (like those optional buildings I mentioned). So they wouldn't be called PWPs anymore most likely, but would exist in some other way.
For the regular decorative PWPs, I bet they will all be furniture. They might have some furniture that is just considered outdoor-only furniture that would be too big to put in a house and therefore only allowed to be placed outside. Some examples of that could be like the Tokyo Tower-like PWP or the giant billboard PWP from New Leaf.
I just hope they make more of those outdoor decorative PWPs more interactive somehow. I've always wished, for example, that you would see a villager sitting in the water of the hot spring PWP. Or maybe if you have the traffic light PWP some villagers will stop moving when it turns red, then start moving again when it turns green (lol!). That would be really cute and give more incentive to use some of those types of things.
Anyway, tl;dr - I don't think PWPs will exist anymore in the same way. They will be either furniture, crafted things, unlockables, or a combination of those things.