next thing will be 200 euro bills revoked, then the base rate can go down to around -1%.
As far as I am aware, among all the Euro countries, Germans are particularly fond of cash.
Since I work in an area that is related to finance (think in the direction of planning/writing business applications for the banking sector), I seriously very often get asked (in Germany) whether one should cash out all the money from the bank account and store it in a strong private safe to avoid the negative interest of the banks that are in the wind.
In effect this makes X% of cash "burn off" every period, and allows the monetary authority to set any negative effective interest rate that they want.
This is by no means an endorsement of negative rates. But if you're going to go down that road, it's a better scheme than just trying to make holding cash too much of a pain in the ass.