The cards you can unlock on Skypass will come from either the latest expansion; or the core collection, which includes all cards released prior to the newest expansion. There will be an icon on the reward which will indicate the type of card you will unlock.
The prism of the card you will unlock depends on the Hero you played during your last match prior to claiming the reward on Skypass.
For instance, if you used Fox during your last match, the next card you will unlock will come from either the Strength or Agility prisms. If you have unlocked all possible cards from these prisms, you will begin unlocking cards from other prisms.
But when do I begin unlocking cards from the newest expansion?
Once you have unlocked ALL cards on all prisms from older expansions, you will then begin to unlock from the newest expansion, regardless of the card type indicated on Skypass.