Essentially, with a post-1900 POD, keep as much of the left as possible as United as possible. What I have in mind is the Second International not splitting, or the split being much less effective. It could need more radical social democratic parties in Europe, perhaps no Russian Revolution or a very different one. Ideally, the challenge is not only to prevent an organizational split, but keep the movement as a whole united, not only in organization but in ideology as well. If you have a good POD aim he very late -9th century, that is also welcome(though maybe the mods will have to move this, idk)