I agree there should be more languages. Definitely, Dutch, Arabic, Hindi, Chinese and Russian. Also some more Central and Eastern European languages. This could make Duolingo more popular in terms of widespread language learning and develpoment.
I additionally would love to learn Icelandic, Finnish, Mandarin Chinese, Japanese, Arabic, Turkish, Russian, Lakota and Sindarin. Sindarin would be stunning.
Oh, and Latin would be fun. Inclusive audio, please. :-) By native speakers...
I think that Chinese (Mandarin) should be available for English speakers as well as other dead languages such as Latin and Ancient Greek because it helps with different languages that are already available on Duolingo