If Russia changed its alliance partner on the Balkan from Serbia to Bulgaria, I would assume it would reduce some of its conflict with Austria-Hungary, which might make it easier for Austria-Hungary to make an alliance with Russia. In that case, Serbia and possibly Romania, would ally with France, Britain and the Ottomans, although it might be that Romania would not dare to take on its Russian neighbour. If they did, and if they were on the losing side, I assume that Russia would as a minimum demand the Dobruja-area (that is the coastal area east of the Danube) and Austria-Hungary might take much of the rest.