A game where you learn a foreign language. Not like actual language learning software, since those all (to my knowledge) have the language learning process work like a game, but an actual game, with an overarching plot, but with learning that language thrown in. Like you're on vacation in say, France, and you end up in some kind of web of intrigue. In addition to solving the mystery you have to figure out how to talk to people. Or you're sent back in time to the world of King Arthur, and have to learn Welsh in addition to performing knightly duties. You can go wild with different settings as you go through languages.
This sounds silly, but the thing is, memory works better when you're trying to engage it on multiple levels, rather than jamming words in there. Of course, the flaw is that people will just remember the plot, and not the words they learned. Still, its never really been tried, outside of a few games teaching English, ironically enough.