The first problem is that the three most "progressive" non-Communist parties--the SPD, the Democrats (left-liberals), and the Zentrum--did not get a majority in the Reichstag even in the 1928 election. To form a coalition, they needed the support of the DVP (right-liberals), and that made any German "New Deal" impossible. Second, even if the three "Weimar parties" did get a majority, it would be hard for them to form a stable government, given that there was an important right wing of the Zentrum, which again would be opposed to any "New Deal." Third, and related to the above, fiscal orthodoxy was very strong in Germany (even among the Social Democrats) where many people had memories of the hyperinflation of several years earlier. Brüning's deflationary policies of OTL still look crazy to me (though they have their defenders--and in any event the fear of inflation was quite widespread in Europe at the time as noted in
https://www.jstor.org/stable/40542979?seq=1#page_scan_tab_contents ), but going to the opposite extreme with a major deficit-financed public works program may simply not have been politically possible for Weimar democracy.