If they are badly beaten, and the Tsar doesn't intervene on their side (and he's still paying the costs of the Crimean War and the Polish Revolt) it could go very badly indeed.
Austria was looking to regain Silesia (in exchange for writing off Lombardy and throwing in Venetia) and creating new Principalities in Prussia's Rhenish lands for the deposed Habsburg rulers of Tuscany and Modena.The rest of the Rhenish lands would probably be shared out between Bavaria, Hanover and maybe Wurttemberg, while Saxony expanded back to her 1813 borders. Of course if Prussia can put up enough resistance FJ may have to settle for less, but if their armies in Bohemia are lost, that would be difficult.