Incentive to keep all lessons gold
I think it'd be useful if Duo were to provide some kind of incentive to keep all your lessons golden. It'd encourage people to practice more often. Duo should grant the incentive if all completed skills are golden at the end of the day (since I'm pretty sure that skills don't degrade intra-day).
I'm not sure what the incentive should be, though. Points? Lingots? A second streak? Some kind of badge? A gold star?
That would be an incentive on top of an incentive. The incentive is that if you practice, your icons turn gold. ;)