Suggestion: Lingot use
I seem to be accumulating quite a few Lingots and I was thinking perhaps Duolingo needs some more Lingot sinks :D
Use Lingots to buy graphics, themes/backgrounds and accessories to personalise your Profile and Avatar?
Use Lingots to buy themed vocabulary packs (funny, offbeat and non-essential) eg. Monty Python phrases, Star Trek vocabulary, common MMO phrases etc.
(Please add more suggestions if you have them!)
Haha this is a wonderful idea!! I would enjoy feeding Lingots to the owl, Language Learning combined with a virtual pet site! :D :D The owl will need a lingot now and then to stay happy and healthy, and by feeding him up you can advance her/him into a more developed Owlie that can... audibly hoot! Dive! Yes and lay an egg! Then the possibilities are endless.
I think it would be great though if you can use lingots for additional features. You earn quite some lingots while moving down your skill tree. It would be fun if you can then use these for more advanced features. As suggested above additional games or vocabulary lists.