Wall Charts of the Language Trees
I'd like to suggest an item for the duolingo gear store* - large, poster sized wall charts of the language trees for all the offered languages.
You could buy one and put it up on a wall and then colour in the various skills as you progress through them, thus acting as an additional motivation tool. It would also be nice to be able to see the whole tree at a glance rather than having to scroll through it on a computer screen.
Also, I know that duolingo is popular in various countries where people may not be able to readily afford the cost and shipping of such an item from a US store. So I would propose that the charts are also made available for free as downloadable PDF files, licensed under some version of the Creative Commons licence. Then people could also print them out for themselves.
* By 'store' I'm referring to the online shop for purchasing duolingo gear that kristinemc recently confirmed is being set up. Not the store on our homepage with items available for lingots.