If you were referring to the public, and if they could express a preference for a specific electoral system, some of our electoral systems in the Länder were changed after referenda, but referenda exist in Germany only at the level of the Länder, so there is not the possibility of holding a referendum for the Bundestag elections.
There is a civic movement to try to change the Bundes law to introduce referenda on the level of the Bundestag. Certainly there is also the intention to use this referendum for a change in the electoral system. But the changes where this took place are changes.... They kept the essentials of the mixed member proportional system, but they replaced the closed lists with open lists.