Jonny is dropping hints over on the groupies chat what the bonus 6th game will be.

So far, here are the clues that he's dropped:
1 - Kan - (some have speculated that this could mean it's "To the Moon". Kan is the first name of the creator of that game.)
2 - Three word title
3 - It's a steam game
4 - Carl - (I have no idea on this one)

5 - It was released in 2012 - (he has since retracted this clue since he thinks he's got the year wrong.)