Extra Points for Extra Time left
If we complete a timed practice with seconds left, how about each second gets turned into one of those nice little golden medal skill points?
If that seems like too much, how does 5 or 10 seconds equaling 1 skill point sound?
I don't think that would be a good idea. You could skip all the long sentences and only answer multiple choice questions and two/three word translations and get 4 or 8 extra points because you had 40 seconds left... Nice if you want to level up fast but it doesn't seem to promote language learning :)
Maybe give the time bonus if you got all 20 correct, or very close to it, like if you got 17+ correct answers and beat the clock.
They can weigh it so that larger sentences give more time and shorter ones give less depending on the number of words it has; problem solved.
Besides, what does leveling up do for anyone? Maybe this is just me, but I've never been motivated to get on duolingo because I wanted to gain levels.