Why would they care about a new map mode. They're gonna push it back to when Riven is implemented.
Mostly because that's all they make money from. Champions. Anyone enthusiastic about new maps is obviously new, and wasn't here for the blatant let-down of twisted treeline, and "shelved" magma chamber.
They just get people to work on these things because making new champions are stressful and they need to work on other projects to get the creative juices flowing.
Edit: Oh, and I forgot to mention. They'll push it back again after Riven because "server's aren't prepared to handle etc etc etc"