The Straftatbestand (§ 315b StGB) exists for all alterations of traffic. After all, you could conceivably start to manipulate traffic lights [1], remove STOP-signs or erect low-visibility barriers.

[1] https://medium.com/asecuritysite-when-bob-met-alice/traffic-...

And this is what would be dangerous. But adding a crosswalk is not, if the law says otherwise it is a bad law.

Adding a crosswalk may be dangerous depending on the location.

A crosswalk is an obstruction of the flow of traffic. There is a reason why crosswalks are announced with traffic signs (Zeichen 350). The fact that you, personally, like the obstruction does not mean they are a lesser problem than a concrete block.

Obviously it’s quite dangerous for pedestrians to use a crosswalk which is not properly marked..