I just can't imagine that Evan Peters is just stunt casting once all is said and done. Feige doesn't tend to do that and I trust that he has a legitimate reason why he's in this series, even if it's not necessarily fully apparent to us at the end of WandaVision.
Although...
I could see a scenario where Wanda is angered so much that she somehow makes Billy, Tommy, Pietro and Vision somehow real outside of the hex.
If it’s a stunt casting the series will go from universal love to one of the most hated things in the MCU. That’s how bad it would be. I can’t see them doing something that shortsighted.