Can anyone think of a plausible scenario by which the "Weimar coalition"
https://en.wikipedia.org/wiki/Weimar_Coalition parties--the SPD, the Zentrum, and the Democrats--could get a majority in the Reichstag in the 1928 elections, so that--at least in theory--they could govern until 1932 without the support of the DVP, without presidential decrees (which Hindenburg would not in any event use to support a coalition that included Social Democrats, as distinguished from a bourgeois coalition reluctantly tolerated by the SPD) and without new elections? One possibility would be a cut-off point by which very small parties--and in 1928 that would include the Nazis, who got only 2.6 percent that year--could not get the benefits of proportional representation. (It is true that if that cut-off point were five percent as under the Federal Republic, the Democrats themselves might be excluded as they got only 4.8 percent of the vote in 1928. So maybe we can make it 3.0 or 4.0 percent.) Another possibility would be that the Wittorf embezzlement scandal in the KPD
https://en.wikipedia.org/wiki/Wittorf_affair becomes public knowledge *before* the election and leads just enough OTL KPD voters to vote for the SPD.
Admittedly, even this would not guarantee Germany stable government, since there was a right wing of the Zentrum that was increasingly wary of cooperation with the SPD. But it would certainly offer better prospects for stable *parliamentary* government (i.e., not resting on presidential decrees) than Germany had in OTL. A coalition stretching from the SPD to the right-liberalism of the DVP would be hard to maintain in the best of times--which, obviously, 1928-32 were not.
For the actual results of the 1928 election, see
http://en.wikipedia.org/wiki/German_election,_1928 The SPD, Zentrum and Democrats combined won 239 seats out of 491--seven seats short of a majority. In December 1924 the same parties had gotten 232 seats out of 493.
https://en.wikipedia.org/wiki/German_federal_election,_December_1924 Thus, after their spectacular showing in the January 1919 elections
https://en.wikipedia.org/wiki/German_federal_election,_1919 the "Weimar coalition" parties were sufficiently weakened by the Versailles Treaty that they would never again quite get a majority, not even in the two elections (December 1924 and May 1928) that were relatively most favorable for Weimar democracy.