Hi! I have been thinking of updating this asset pack with more flags for some time. It's just really hard to choose which ones. The current pack plays it pretty safe by only including United Nations Members and observatory states, but I'm aware of how that leaves important territories out, like Hong Kong and Macau, as well as widely used flags like US States, Dependant countries and other territories (Scotland, England, Greenland, Antarctica, etc)
Perhaps a good approach would be to copy the list that UNICODE uses (the flags accesible on the emoji list of your phone) but that means making a lot of new flags, which will take me some time.
I would love to add Hong Kong but it would be strange to update the pack for a single flag. I will figure this out and update the pack eventually, sorry for the inconvenience.