Nitrome is having a lot of games which don't get released to public. Examples enough of games we eventually got to see. Because I know that graphic designers do release some of their artwork which is not being used in games, it sounds an interesting fact to discuss their artwork.
In the following blog post, there's a mock up for a top down view miner game... Aside the picture, there's nothing much more given than the title of this blogpost and the word 'possible'. http://markusheinel.tumblr.com/post/70578735333/quick-sprite-mock-up-of-a-miner-for-a-possible-top *
Could it be a game that Nitrome thought off? Of course, this is just based on a picture, and so good as speculation, but still... During 2009, Markus did the artwork for Cave Chaos, and 3 years later he did Cave Chaos 2.
(* I want to warn that the above link is clear of adult-only pictures, but the blog may contain mockups which have an adult rating)