I also heard someone say it is an untold segment of the original story, but it can't be, because of the continuity issues. Hughes claims to meet the Elrics for the first time in the first episode of the Brotherhood series, he is a Lt. Colonel, and his daughter is already old enough to talk coherently (3 years old?). All this stuff didn't happen until well after the 13th episode of the original series, and the Elric Brothers already knew Hughes from before his daughter was born.
Then I should say to you that Ed and Al didn't meet Elysia before she was 3 years old in the manga (in fact, Winry was the first to meet Elysia and Gracia, not the Elric). All this stuff was invented for the anime.
The Ice Alchemist can be considered an untold part of the story, but you have to know a lot about the manga, up to the 15th volume...
