What if Bismarck decided, consistent with past reciprocal good relations with St. Petersburg, to endorse Russia over the controversial San Stefano peace treaty (resulting in the "Big Bulgaria", playing the role of "honest partner" rather than "honest broker"?
----assuming a Big Bulgaria, what is the future fate of Serbia and Habsburg-Serbian relaions?---
With German diplomatic endorsement, does San Stefano simply become a fait accompli?
Or, do the British and Austrians war with Russia to revise the settlement?
Who does France side with?
If this escalates into a neo-Crimean war (with Britain and France and Austria in an anti-Russian coalition), do those powers also take a punitive approach to Bismarckian Germany as well?
What other alternatives are available besides conflict?
If San Stefano creates a pro-Russian fait accompli in Europe, might other powers take additional Ottoman territory in Europe, Africa or Asia as "compensation" (like Cyprus and Bosnia in OTL, only more) justified by balance of power?