I have a problem with all the apps/sites that promise you that you can learn a new language: They are mostly vocabulary trainers, not language trainers.
Simply learning some vocabularies doesn't cut it, a language consists of much more. For example in duolingo: While I learned some vocabularies, I have no idea how to create a sentence or if there are rules for the accents in spanish etc.
I wonder if there's any particularly good way to gamify learning grammar, though— you can't really quantify it in the same way as you can vocabulary.
My favorite suggestion for online language learning, whose source I've long since forgotten, is to have a story / article / other piece of writing that gradually transitions from English grammar to language-being-learned's grammar, then performs a similar transition over vocabulary (like http://dotsies.org/stories/the-lamplighter.html , but with an alternate language instead of alphabet).
I haven't seen any implementations of this, and it'd probably be practically impossible to automate, but feel like it has some potential.
Yes, the method is simple: massive exposure to input (see the Assimil "without toil" series for a good example). Using spaced repetition and NLP you could introduce gradated input customized to the user to provide ever increasing vocabulary and grammar points. This is a practical version of what your dotsies example would look like, individually adapted to users.
I've actually thought long and hard about how one would construct such a site and what the algorithms would look like to provide effective language learning. It's just one more thing I don't have the time to actually do, however, as much as I would love to actually use the site...
EDIT: If someone actually wants to do this, email me.
Where might I find your email? I've thought a good bit about this too, I actually bought extensivereader.com a few years ago as a placeholder when I was experimenting with building something, and have recently been considering attacking it again.
The approach of the Michel Thomas Method is the closest thing I've seen to teaching grammar in a way that directly exercises our grammar-learning ability. In their audio, there is a teacher and two students. The phrases and words you are taught are not anchored in practical usage the way Pimsleur does, but are more random. However, the exercise portions require you to construct completely novel arrangements.
That is, let's say they taught you how to say "I want a cookie" and "She hates cats" you would be asked to say "She hates cookies".
In schools they use tests to quantify knowledge of grammar. Forget about grades, collect eggs, wooden sticks or whatever, and there you have your gamification.
I paid for a couple of months for the chinese version, it's really well made but the content is not exciting enough. Can't speak for other languages but I really think this is the best way to learn (this and podcasts).
Simply learning some vocabularies doesn't cut it, a language consists of much more. For example in duolingo: While I learned some vocabularies, I have no idea how to create a sentence or if there are rules for the accents in spanish etc.