I really want the team to work on an icelandic course on this site, it would be pretty nice since its mostly the English alphabet and pretty similar to norwegian and swedish, also I've noticed duolingo isn't very good for grammar, I don't know if the icelandic grammar system is much different from norwegian. (I'm not a language expert I'm just expressing my ideas)
I believe an Icelandic course request has already been posted.
I wouldn't learn it myself since I have no use for it (other than having a friend from Iceland that speaks considerably okay English), but it'd be cool if Duolingo had it. I've yet to see a good place that teaches Icelandic, albeit I haven't really looked for one.
Icelandic isn't too similar to Norwegian and Swedish, other than both being related to Norse and being Germanic. I'd compare it to Spanish and French, similar but many differences. I've heard the grammar in Icelandic is super difficult.
I've also heard Icelandic hasn't changed very much over the years, and mutually intelligible with Old Norse, which is cool.