Suggested Course: Toki Pona for English Speakers
Toki Pona is a conlang developed by Sonja Lang in the early 2000's that was designed to push the minimalistic boundaries of language use. With a lexicon of only 120 words, Toki Pona is meant to distill the essence of what we communicate into its most simplistic terms. The inspiration for the language's development comes from Zen philosophy and Taoism.
Since Duolingo has created courses for conlangs in the past, I thought Toki Pona would be an excellent (and hopefully simple) addition to the list!