As mentioned, others have been in but as very background characters with very few lines if any. The Sheeran spot felt forced, almost like a SNL sketch or something. I know in the books Arya does come across a singing soldier so it fits but he didn't need to go "yeah, it's a new one". One shot of his face singing would have been enough to show he was there like the other "guest spots"
Anyway, it's done now