err...trade the US dollar index futures maybe? I don't know how it solves your issue, because instead of one asset you will have multiple currencies influencing your dollar trade....but I mean...If you just throw something at the wall and hope that it sticks you might just do it with something you perceive as simple.
But the dollar is 50% USD vs. 50% "basket."
Whereas USD/JPY is 50% of each. It's just as much moved by the other random currency I chose.