A interestng change to holidays would be, instead of an instanced bosses, to have zone events. They could be phased for the level bracket of those zones only, where you deal with a big invasion and an eventual big boss. This will open up the possibility for lower level players to experience the event to it's full extent and to make people go out into the world again.
I would add a solo scenario for some, if not all, holiday events. Examples of scenarios:
Christmas scenario- Objective: Help Greatfather Winter to deliver christmas gifts! How: Fly on a reindeer mount (or a special made mount for this quest) and right click chimneys in one of the main cities.
Halloween scenario- Objective: Successfully trick-or-treat 5 times. How: In this scenario you choose an outfit to go trick-or-treating in. Knock on doors and cross your fingers!
I think this would be fun. Different enough to be scenarios, short enough to be dailies.