So if you want to see if your favorite script works out of the box on the new page, you can edit the script to activate on WaniKani — Log in
I know there was quite a bit of concern about that.
Leech tables and Self Study Quiz, at least, work right away with no other modification. So if you don’t want to mess with it, at least know those will work when the preview site becomes the main one. Ultimate Timeline is already working for me on the preview (thanks, @rfindley)
To try it, figure out how to edit your script. In firefox, click the tampermonkey icon, choose dashboard, and the script will have an edit button. At the top of the script, there’s a bunch of comments, and one should be
// @include WaniKani — Log in
add another line just like it underneath that with the new URL, then file:save. Reload the page.
Edit: Wanikani Heatmap works too.