[Steam 2018 Spring Cleaning Event] [Predictions] What's in the box?

Having completed 5 of 18 trophies (reaching tier 1 of the Spring Cleaning badge) granted the Steam user a Spring Cleaning background, 3 emotes, and a mystery box. Description of the box states:

This item might be useful in a future sale.
Earned for participating in the Spring Cleaning Event in 2018.

Any guesses? Personally, I predict the mystery box will relate to Valve's new card game, Artifact, and perhaps Steam users who have the box may obtain an Artifact card or a card pack. Why cards? Well, Valve has released items with price tags for other games the company has developed. CSGO has skins, Team Fortress 2 has the hats, Dota 2 has market items on the community market (I am vague on what Dota 2 has). I cannot deny Artifact might have marketable cards in the future. Even though the description mentions the box might be useful in a future sale does not indicate the item must relate to a sale; just that an event coinciding with a Steam sale is possible such as if Valve released extra information about Artifact during the summer sale for example.

Edit: To add to my prediction, Artifact on the Steam store has a 2018 release date.
2018 Spring Cleaning event page
Spring Cleaning market items
Mystery box listing

Edit: The Mystery Box contained cosmetics for your alien in the Summer Sale 2018 minigame called Saliens.
https://steamcommunity.com/saliengame/play/