I would absolutely love a "Strengthen Skills: Endless Mode". It'd be handy when you're revising a tree you've completed but neglected. Like the Temple Run of language learning. What do you think? :D
I'd love it! Especially if it was for the whole tree and not just one or two skills (and that doesn't even seem to work at the moment, at least for me it's one skill only). As it is I can guess pretty soon what grammar the button wants me to study and I know what rules I have to use for all the questions and I start to automatically use the past forms (or whatever the skill is). But I don't have to think about the tense of each sentence and it's not as big a challenge as it could be.
There was actually a duolingo userscript created for this very reason a long time ago so the idea isn't anything new. However, the script stopped working a few months ago.
It is a good idea nonetheless, because it's a pain in the behind for me to keep refreshing the page just to practice again.