The two 5-packs of Kreons that come out this year, which feature on the boxes a black silhouette of the fifth figure. Now that these have arrived at an online store, we've been shown what the mystery figures are. (which could help people decide if they want either set)

The First set has Optimus, Sentinel, Megatron and Soundwave, with Starscream as the mystery figure.
The second set has Bumblebee, Red Alert, Mirage and Jazz, with Skywarp as the mystery figure.