To me the US dollar definitely seems to be on its way down in the medium term. Here is why:
Currently lots of the value of the dollar is based on the petrodollar deals the US has made with the big oil-producing countries such as Saudi-Arabia (i.e. military protection and weapons for agreeing to only accept dollars for oil). This has resulted in a big demand of dollars all around the world because also those countries not directly involved in these deals need to have dollars if they want oil.
This big global demand for dollars has justified and enabled the FED to increase the dollar money supply without diluting the value of the currency. If any other country that only had a domestically used currency wanted to print as much money as the US has done it would result in immediate massive inflation with obvious negative consequences. However, the US has been able to do just that because of the large international petrodollar demand.
What has been going on during the past decades is that US has been using all its military and economic power to preserve and boost the petrodollar system. There are constant economic sanctions and political pressure against all small-to-medium-size oil-producing counties (like Venezuela) who don't participate in the petrodollar circle and there have been wars fought to preserve the system. For example US attacked Iraq just after Saddam switched the oil trade from dollars to euros. Naturally the first thing they did was switch the Iraqi oil trade back to dollars. Currently there are factions in the US political scene drumming war against Iran (one of the few big oil-producers who don't accept dollars).
All this has been an artificial effort to boost the demand for dollars. I don't think it can continue as it is for very much longer. The US is losing its grip to unilaterally dominate the rest of the world and everybody is becoming less dependent on oil (alternative energy sources like solar are becoming exponentially more efficient).
What will happen when all the countries that have been hoarding dollars in order to participate in the oil trade don't need (so much of) them anymore?
The laws of supply and demand cause the international exchange rate of dollars to collapse. Where will the dollars go? They will naturally flow to the one place where there still is demand for them: back to the US. So the US domestic money supply will quickly and vastly expand thereby collapsing also the domestic buying power of the currency. The world reserve currency status will be lost.
I don't know what will be the next reserve currency. I don't know if we even need to have one singular reserve currency. With modern technology all currencies (especially cryptocurrencies) are easily and quickly exchangeable, reducing the need to have a common currency.
EDIT: related links:
https://en.wikipedia.org/wiki/Iran%E2%80%93Pakistan_gas_pipelinehttp://www.gulf-times.com/eco.-bus.%20news/256/details/398622/kuwait-finance-firm-suggests-trading-oil-in-bitcoinshttp://www.forbes.com/sites/energysource/2014/05/29/the-colder-war-and-the-end-of-the-petrodollar/
