Fable: Heroes is an XBLA game due this spring, although we're not actually supposed to know that. It was apparently announced at Microsoft's Spring 2012 Showcase in San Francisco, then reported on by Gamasutra in a piece that has subsequently had all reference to the game removed. This can only lead us to the conclusion that information about the game must be under embargo.
No more details were announced – apparently the Gamasutra piece only mentioned the game by name, anyway – so we're not quite sure what form Fable: Heroes will take. It being an XBLA title doesn't really mean much in this day and age and it's anybody's guess in which direction this will take the series.
Fable 3 wasn't especially well received when it first came out: it had its good points but the last portion of the game was very underwhelming. Now that the cat is out of the XBLA bag, it shouldn't be too long until we get some actual information from Microsoft and Lionhead.