When are we going to get more content?
Now that I'm closing in on the bottom of the skill tree, when are we going to get new content? Whether it's purchasable extras (like French skills you can buy in the Duolingo store for lingots, thus making lingots actually useful) or just an extension of the tree, when will they be available? Has this typically happened for other courses?
Many courses have been updated with new versions and additional content, yes. This can happen once the course is out of beta. However, it it typically the work of many months, and then it can be many more months before it is universally accessible.
Your best bet for further Japanese study on Duolingo is either furthering your mastery of the content on this tree with the new skill levels or using the English for Japanese speakers course.
a) Marugoto course: https://minato-jf.jp/CourseDetail/Index/KC17_MGRS_A101_EN02
b) Lingvist also has a Japanese-to-English course (requires free rakuten account).
The course is still in beta so it could take quite a while. They still have to work out all the bugs in the content they already have. Though it does feel like it's updating slower than other languages, the courses are created by volunteers so there isn't really a time estimate at all, just when people can work on it.
You can try the reverse-tree method for now or look at other sources for learning. I personally use Linguti which was created a few years ago for a lack of asian languages on Duo. It has a similar structure but its quizzes are separated by vocab, phrases, grammar, reading, writing, spelling and listening. There's also Japanesepod101 and multiple youtubers who do lessons which gives you a much clearer explanation of grammar concepts.
Ways to keep your learning progression while you wait for a Duo update.
I am basing my problem with DL Japanese with my experience in DL Spanish. DL Spanish is responsive, pays attention to comments and discussions and gives feedback. DL Japanese seems never to admit that other translations might be accepted, and is not only rigid in what it accepts, but many of its 'correct' answers are in very strained English. When I get e-mails from the DL Spanish discussions I can go to DL and look at the discussion. When I get e-mails from DL Japanese , I get a 404 error notice. For a project in Beta, DL Japanese is exceedingly unresponsive to comments, suggestions and discussions. How can they make it better if they ignore the users?
Hmm. I'm not entirely sure if this is a matter of them being unresponsive. I do get email updates that they've accepted different translations I've suggested, and I've never had this 404 error occur. These might be different issues. Honestly, you might consider posting that in the Troubleshooting forum.