The Stranger Things set will be mechanically unique cards that will be available through a Secret Lair drop, and then later as in-Magic universe cards. These cards were specifically designed for use in the Commander format, and are not intended for competitive formats. Thus far, the only image spoiled for this set has been artwork of Eleven using her powers against an Upside-Down background.
Wizards of the Coast recently released some of their thoughts on how to handle potential card availability issues with the mechanically unique Universes Beyond cards, which will be sold through upcoming Secret Lairs like this one (see "Wizards Rethinks Their Approach to 'Magic: The Gathering' 'Universes Beyond' Releases ").