Duolingo app should allow users to download unlimited lessons.
If people could download as many lessons as they wanted it would allow them to do a lot more when not connected to the internet...
I don't have mobile internet but after a while I stopped using the pre-loaded lessons. If you make enough mistakes and have to start over, it just gives you the same sentences in the same order, which I feel is not so much learning as memorising the answers. Then again, pre-loading more sentences than necessary just for the sake of randomness would also take up more space on your phone, so it should be optional.
Memrise has that feature, and it's definitely come in handy since I have unlimited internet at home, but very little mobile internet.
It doesn't really have the functionality OP is looking for. As of right now, it only pre-downloads about an hour's worth of lessons. I'm going to be traveling abroad without Internet for 3 weeks, and would like to be able to pre-download the whole course. . .