I agree this would be best served by having a separate track for the existing emacs snap (perhaps called snapshot since this is familiar to anyone using the existing Elisp PPA Ubuntu Emacs Lisp : “Ubuntu Emacs Lisp” team) and to maintain this as a branch in the emacs-snap repo (https://github.com/alexmurray/emacs-snap/) - I would love to collaborate with you on this 
I am not certain how this would work with the build.snapcraft.io service but I am sure we can figure it out - so that there is a single emacs snap which offers both a stable build and a build which tracks git master etc via the tracks mechanism.