So many people farm event items that the demand is unlikely to go up much. We have Canthan New Year approaching, and after that the Sweet Treat Weekend, Lucky weekend, etc etc.
In my opinion, the only items you see gain value as we get further away from the event where they drop are pcons like apples, corn, golden eggs and pumpkin pie. Speedclear people burn through a ton of those types of things, so there's always a demand. Normal stuff like alcohol, tonics, etc usually stay at pretty much the same value.
And yeah, I think you're being fairly optimistic about a demand spike when they set a date for GW2. I'd say that most of the people who care about filling their hall started working on the consumables titles when the calculator was released.