There are a lot of Americans who have their primary residence overseas. Where does their presidential vote get counted? Can ex-patriots in places like Europe choose a state to register in? Can they only vote in Federal elections?
What about people who live in U.S. territories? During the presidential election all the focus seems to be on this swing state and that state, never any territory or such. I never hear about presidential candidates travelling to Puerto Rico, for example - and that island has more than 3 million people. By comparison, Missouri has 5 million + and you always hear about Missouri.