I think there could and should be options to actually merge SPD with other parties.
Dynamic (I think) allows for creation of SED in a left run, but not during gameplay. Likewise, I came to a conclusion that really right-wing SPD (full-on reformist, with left and center split off) should be able to conjoin itself with other parties.
We can "help" DDP if it reaches low popularity, but I came to an idea of a Great People's Party (dubbed Soziale Volkspartei in my mind), a merger of SPD and Zentrum (under Joos or Kaiser), along with (maybe) other Weimar parties, as a vanguard against extremism.
I don't really believe it had any historical precedence, but it's an alternative history game afterall.
I think of requirements like this:
- Reformist SPD without left and centre, with neorevisionists unlocked and People's Party achieved
- Joos/Kaiser Z
- Left-wing DDP/LVP
- Low popularity of the Weimar parties
- DNVP collapsed (either Treviranus KVP formed or Hugenberg's NVF formed)
- High popularity of KPD and NSDAP
- Electoral thresshold passed
- Concordat with the Vatican signed on Prussian level and no radical women's rights passed (as it already has negative impact on national Concordat progress)
And of course, the SED being unlocked during gameplay would be a very nice addition, removing the KPD from the polls and the political sphere and fulfilling the Left Front.