I read that idea about the Sultana showing up in the Goblet and I thought it would be a great thing for all the instances. Merlwyb showing up in Mist, or maybe the Elder Seedseer showing up in Lavender Beds.
Maybe have other random NPCs walking around? The sylphs making a pilgrimage to Gridania to leave presents for walking ones? It could be anything really, so long as it makes things a bit more lively.