One additional combatant would be Austria on the side of the Western powers. Count Buol primarily wanted to dislodge Russia from the lower part and mouth of the Danube and forced them OTL to remove their troops from Moldavia and Wallachia. AFAIK, he never intended to enter the war.
But Tsar Nicholas might call his bluff and things might escalate. If you have the assassination attempt against Franz Josef succeed in 1853, then an emperor Maximilian might explicitely wish to ally with the "progressive" Western nations and fight against Russia.
This will have of course have big repercussions in the German Con, since Prussia did want to avoid war and many smaller dynasties had friendly relations with Russia. Austria might gain diplomatic clout with London and Paris, but lose prestige in other parts of Germany.
But of course an Austrian participation in the war would be a disaster for Austria. With the Magyar tax boycot and many other financial crises they were near bankruptcy. Whatever support the UK and France give, Austria would bear the brunt of the fighting. In exchange for this support, it might have had to accept any demands for its future policy in Italy, the Balkans and elsewhere.