The recent town transformation (not the overworld, but inner) for St. Patty's Day was wonderful. If we had simple cosmetic changes for gold, no boosts or advantages inherent, I believe this would be a good source of revenue for Plarium. There's already the Hero's profile changes, and also the overworld's town changes (but we could strip the cosmetic change to be separate), but what about other potential replacements?

Potentials:

Overworld avatars (hero and shaman)

Equipment (all types)

Buildings (if not straight different, then able to choose a prior level's look)

Town, Inner (like St. Patty's Day's looks)

Town, Overworld (like the Warrior's Fortress) 

Town, Overworld, those that are currently inactive/dead.

Town, different idle animations for buildings, extra folks, etc. 

Town expansion (how far we can scroll in all directions) [this also has potential to allow for actually larger cities, though I digress] 

Stronghold, Inner (like St. Patty's Day)

Stronghold, Overworld

Stronghold, buildings

User Interface (if not straight different, then at least other colors)

Overworld map

Music, all areas

Sounds, all versions

Aesir, visibility in the town.

Aesir, visible changes when active.

Et cetera.

In short, there's a lot of room for growth here, from both sides of the coin.