The most convenient thing for users would probably be to make area downloads available by zoom level (download either a city, the state/province, the whole country, or a whole region in some cases). I’m not sure if the app is already doing this in some way, but it isn’t always obvious.
For example, when I tried to download map data for Guam, I would have wanted to download data for all of the Marianas islands at once, or maybe even all of the Micronesian and other Pacific islands as well, since they are so small. But the app restricts downloads to one island at a time, and the download for Guam was labeled for the Guam village of Dededo (lots of other villages exist on Guam, but nobody would ever just download map data for Dededo!).
Suggestion: make map data downloads available by a set number of larger regions — North America, South America, Europe, Asia, Africa, Australia/New Zealand, Oceania. Then also make it available by country or state/province. Then by individual cities as well, if need be.