I'm particularly intrigued by today's Shakespeare Apocrypha play, Thomas of Woodstock, as it is, essentially, a prequel to Richard II.
Richard II picks up almost immediately after Thomas of Woodstock, with people arguing over who murdered him. When I saw Breath of Kings, which condensed Richard II, Henry IV parts 1 and 2, and Henry V into two plays, at the Stratford Festival last season, they actually opened with the murder of Thomas from Thomas of Woodstock, which I thought was a nice way to try and fill in the backstory of Richard II a bit.
Thomas of Woodstock exists only as an incomplete, anonymous manuscript. Many people have tried to argue that Shakespeare wrote it, but, as with the other apocrypha, there is no evidence and no scholarly consensus to back up that claim.