Actually, I think I've been bs'ing in my answer. I think the real reason I would want Singing Mountain is mainly because of the song. For some reason, cut songs from games I'm big on, and I always like to know where they went. So I would have wanted it implemented somewhere.
As for 1999 A.D., it would be nice to play in it, say Lavos doesn't erupt for awhile, or while his attacks take a long time to do and you can dick around on the world map or whatever. However, I think that would take away the sense of urgency... then again, they find out about Lavos at the beginning of the game and seem to be taking their friggin time.