Your definition of Canon is right on (and supplemental info is just that: supplemental.) Fan fic is inherently ex-canon. Speculation and elaboration inspired by (vs slavishly adherent to) canon makes for great stories, IMO.
I also agree whole-heartedly that the Headmaster!Snape time frame holds a lot of opportunity for great stories. (*nudge*)
I can't wait to see all the fic generated by these Games :D