They have an update for arabic in the Incubator! Take a look here: https://incubator.duolingo.com/courses/ar/en/status. They also have some new contributors.
From what I understood, they're going to base the Arabic course on the Egyptian dialect. The Egyptian dialect is basically the easiest (maybe I'm biased) because how progressive it is with the time. Add to it, that it's the most common dialect given those who speak it are around or above 100 million speakers.
A big clue is that the course description is using a picture of the Sphinx and the Egyptian flag. https://www.duolingo.com/course/ar/en/Learn-Arabic
The Moroccan person should understand the Gulf person more than the other way around, and as long as both people consume pop culture they'd both understand a reasonable amount of Egyptian (possibly some Lebanese as well). Someone from Morocco would have an easier time understanding someone from Algeria/Tunisia. I've never studied it but based on traveling, I've found just basic things like "how are you" and "pretty" are entirely different words, and while everyone understands "thank you" and "you're welcome", there's local ways to say those too. Anyway, I personally hope it's Egyptian :).
"The course is going to teach a spoken variety of the language based on Modern Standard Arabic (MSA), though it won't be as formal (or as complex) as traditional MSA."
So, is this gonna be a watered down version of MSA or something? No cases, no alternate pronunciations for words, etc?
Or does "based on MSA" here is referring to a dialect of spoken Arabic? Like Egyptian or something?
Try this website. It helped me a lot when I first started learning Arabic seriously.
It gives grammar lessons in note form, but not just that though. It offers lessons in both Standard and Egyptian Arabic. Some lessons are only for one of the two, but try to learn both since you won't be limited to just one form of the language.
Oh, it also have vocab in both. A lot of it too at that. If you're a beginner in Arabic, I really suggest using this website. While there are some errors in the vocab(typos, you know), it's largely reliable.