Ripple in my opinion is the absolute best digital currency when it comes to use case. Almost every transaction can be improved by ripple. You wanna cash a check from a bank you don't use, at your bank? You can wait 3 days or use ripple. This use case has been largely ignored. With the introduction of amazon and uber people are starting to believe.
To me the international transfer market is still extremely interesting. You can get some idea on numbers here. The daily turnover in fx markets is 5.2 trillion. If xrp can take 20% of those transactons thats over 1 trillion a day. There is no reason this won't happen. Xrp offers better service for banking customers with smaller fees. And banks can easily move their own money around with virtually no fees. Banks will even be able to give loans in markets with different currencies. And, except payments in any currency.
All of the other domestic transactions will dwarf the international transfer market. Their is essentially no limit with Amazkn, Amex, Uber and, all of the normal probles with banking either caused by time to wait for verification or, large fees. All these addressed by xrp. Everyone seems to wanna price predict. All I can say is way, way, higher than it is today.
In my opinion this will be the most used coin by far. If you can save banks money and draw business to them, they will use it. Nobody questions if banks care about anything other than money. So, if you believe banks are greedy heartless institutions, you better believe xrp will be huge.