The new format is a ZIP containing a bunch of JSON files per page, so maybe someone will come up with a cross-platform viewer or similar.
Edit: I found this which works with basic Sketch files: https://animaapp.github.io/sketch-web-viewer/
see also https://github.com/xaviervia/sketch2json for some interesting stuff :)
Most of the slickness of Sketch is in its UI though, it'll be tough to replicate. Do you know of any project attempting to do so?