If you had temporarily gained control of a creature, it won't return to its previous controller
In fact, a good planeswalker can be worth more than the entire cost of a drop, with the best of the bunch (Teferi, Time Raveler, Jace, Wielder of Mysteries, and Liliana, Dreadhorde General) selling for $60100, while even the cheapest are in the $5 range
The right color for you will find its way to you eventually
There's also a "Wonder Pick" mode that lets you copy the contents of a friend's booster pack and pick one card at random to add to your collection
But there are now, and there will continue to be, people who add to it because the work is that massive and that important
Then all the chosen creatures are sacrificed at the same time