Context: I'm a full-time Debian user.
I firmly believe that the future is not around packaging as we've been conceiving of it for the last decade or so, but around packaging in the form of containers[0]. As a Debian user, I'd be really happy to see Debian keep an eye towards containerization as a a first-class citizen of the distribution, the way apt(itude) and dpkg are now.
PPAs can either fit into this model or work against it, depending on how you look at it. They can either enable the creation of containers by virtue of being more flexible and more easily used inside container builds, or they can serve as a crutch for low-quality packaging standards. So, I'd be excited for PPAs in Debian, but I'd like to see them adopted as a tool to facilitate first-class containerization in Debian, rather than the way they are used in Ubuntu more as a place to hold unsupported or less-supported packages.
[0] Not necessarily Docker or even Docker-like containers, but containers nonetheless.