Well, see, the Junker Queen was introduced as a background character way back in 2017 when they made the Junkertown map. She was originally just a function of the map with voice lines playing over intercoms as you progressed the payload, and was simply called "The Queen" on one poster you could find of her on the map.
People got attached to the idea of the character, started speculating and making fan art, and she gradually got built up with minor appearances in other stuff, like Wrecking Ball's origin story video:
By 2019 they had apparently started actually iterating on her character design and planning to turn her into a fully fledged playable hero. And now here we are with her being one of the marquee new heroes of OW2.