All the other game modes could/should have their own themes reinforced with their own minions, monsters and buildings. (Don't forget the brushes.)
Twisted Treeline could have this spooky, dead and corrupted architecture and theme. Buildings are but tainted ruins, and twisted minions are dressed in withered cowls and armed with thematic weapons. The monsters could use more differentiation from Summoner's Rift, along with their Smite Bonuses and summoning animations. I don't know, the jungle feels pretty peculiar to me in this map. Maybe it could be expanded some how. As for Vilemaw, it could be brought to Baron's level when it comes to combat mechanics, giving it this more epic monster feel. It currently is quite impressive, but it could use some pattern.
As for Howling Abyss, the turrets already have this unique, thematic architecture, along with the destructibility. It could be implemented to the Inhibitor and Nexus. Minions could be accustomed to the cold and the place, with their clothing and weapons.
Crystal Scar is obviously the biggest offender, it hasn't been touched for quite some time after all. The whole map could use the update, similar to the others. Currently I feel it doesn't tell its story. The score system needs an overhaul too, to better reflect players' contribution.