There is no killer app. There is killer functionality very much needed.
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preinst + postinst functionality
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user account addition
For a kiosk like application I’ve been experimenting with migrating a .DEB to one or more snaps. With such an application you need the ability to create a non-priv user then to auto-launch an application for that user. This requires writing to the .config directory of said user, once the install creates the user. Normally, with a .DEB, you do stuff like this in the postinst proc because now you’ve unzipped everything AND you know the architecture.
preinst tends to be used for determining the architecture and which portions of content will be “activated” (for lack of a better term).
The current snap universe seems to be focused on the rather microscopic universe of a single pre-existing user install an app (like a text editor) from a store for themselves or perhaps all users. All of the requests I see for postgresql aliases seems to bear that out.
The ability of snap to completely replace Debian packages in every way, including letting 32-bit executables run on 64-bit hardware is high on the wish list of many.