instead of thinking of all the countries that could join the central, it is worth noting which countries could stay (more) neutral
I already mentioned the US, who's staying neutral would me disaster for the entente
Italy staying neutral, instead of joining the entente would make a difference already.
I'll add Romania, if the Brusilov offensive had been a failure or at least not as big a success they would stay neutral as well. That would save Falkenhayn's job with consequences including the Hindenburg program, unrestricted submarine warfare and German ambitions as he did not believe in total victory anymore, unlike Ludendorff.