Ethereum allows to create many sub-currencies. So first create the e-BTC (ethereum-BTC) sub-currency and convince people to use it. Now let's suppose it becomes as successful as BTC and millions of users have a eBTC "account" and millions of eBTC transactions occur every day.
Obviously in a multi-currency world we should also be able to do the same for eUSD for example of convert between eBTC and eUSD (or whatever you may need) so the number of transactions may be even higher (without taking into account the challenge of knowing the current conversion rate).
Does anyone has already estimated (or any indictation of) what would be the effect (load or storage constraints for example) on the ethereum network ?