The April brand store calendar for the USA is now on the LEGO server. It confirms that 30603 Mr. Freeze will be available during the month but more interestingly it provides us with our first glimpse of the 21305 Maze Ideas set.
I have a feeling that the set will be officially revealed at LEGO World Copenhagen which has traditionally been a venue for set unveilings. There's an evening event going on at the moment so it could be later today... (Update: it wasn't.)
You can see the calendar in full after the break.