[dsr_]

No, just leave a blank space in the middle that has nothing to do with the denomination and is there just to be interesting.

Print something new in it for each member; issue half of them in odd years and half in even. Hold contests.

[euroderf]

But then perhaps your photocopiers would need constant updates - to do the usual banknote detection and refusal to copy ?

[kiliankoe]

As someone with the detection mechanism tatooed on their arm, I can tell you this is not based on the visual design of the entire note, but on a small constellation of five rings, the so called EURion constellation [0]. It's also used by other bank notes around the world, inlcuding the Yen and US Dollar.

https://en.wikipedia.org/wiki/EURion_constellation