It's the US, we put presidents and Founding Fathers on our bills. Harriet Tubman is neither, so of course we're not putting her on the twenty.
Instead we should compromise with historical precedent and put Obama on the twenty - he's both a president and a black historical figure and he'll piss off the people pissed off by the idea of Tubman on the twenty even more. It's a win all around!
The US doesn't feature Harriet Tubman on the $20 because it's full up with racist, slavery-denying fools who still have the mindset of an Andrew Jackson voter and would vote a wheel of racist cheese into the office of president over an eminently qualified black woman any day of any week.
Wait wait wait... are you saying you don't have any commemorative coins in the USA?! Holy shit I never realised! We have special coins all the time in Europe. My kids absolutely love when they find a special 2€ coin with some cool engraving on it.
How sad! You could have so many, too! By default each country in the Euro zone has its own heads, while tails are always the same. Easy peasy!