Live challenges of friends on-line?
There are lots of possibilities! 1) Two or more friends are on-line at the same time. One opens a challenge. Others accept. The challenge is to take a timed drill, each in own language and own level. Winner by points/coins/etc earned.
I posted a similar idea as a reply in a "meetups" thread: "If duolingo organizes/runs meetups then maybe there could be special competition drills that only work at a meetup. For example, computers networked at the meetup could all take part in a timed vocab drill, each person in own lang at own level. Have heats so that five or ten or so compete at once (assuming large attendance). Winners from each heat based on coins/points/lingot earned. Winners of a large meetup get a special prize. Would encourage meetup attendance, too."
2) Back and forth challenge -- easiest if one-on-one. This could be live for more fun, but does not need to be live. You could challenge someone, and they reply when they are on-line, and so forth. The first person gets to choose what question will be asked of the second person. Duolingo would offer three or four questions, you would choose one of them, and the other person would have to answer. Here what's cool is that when you want to offer a challenge, Duolingo would tell you what languages the other person has studied, and their current level. Then you could challenge them at their current level, up to a few levels back, or at a level above! If challenged above your level you would get more points for each right answer. If challenged below, then you would get fewer points.
No one would have to accept an initial challenge. So you could choose what to accept and from whom. If you accept someone's challenge, then you not only reply to their Q, but you send them back a question. And they have to accept. Creates a little excitement that way. Also stops challenges from people who do not want to answer.
You could give points for every right answer. You could take away points for wrong answers. Lots of choices. I think it would be lots of fun.
The pace of this type of game would be more like playing chess long distance, where you send your move by email, text, etc.