Let's just say that Hugo Preuss has a change of heart when writing the Weimar Constitution. Instead of full PR, he introduces a threshold of, say, 5%. If a party gains less than 5%, that party would be excluded from the Reichstag, but not so in a Länder's Landtag. This precaution would probably exclude the Nazis from 1924-1930 (if they manage to get in in 1930 at all). So they can't use parliament as a platform.
Any consequences for the Republic?