I guess it is my turn to contribute. So we have had . . .
1) Alternative Marines (Dark Angels)
2) Alternative Genestealer deck (Tyranids)
3) Alternative Missions and Locations (Mission Pack)
I guess this just leaves room for an alternative / add on Event deck. If I had any big idea's in this regard, I think some sort of "mini game" type events would be great. For example, a computer hacking event where you get to add support tokens based on the dice rolls. Or a shooting gallery style event where you attack the blip piles directly but risk being attacked by the blip pile at the end. The above are rough and probably not balanced but you get the idea.
I have only just noticed that other people have suggested this same thing, I might be beating a dead horse but I will put it out there regardless.