With POD as far as 1821, how could the Russian Empire hold an area roughly equivalent to today's Bulgaria, or better the greater Bulgaria included in the Central Power's war aims ?
Also, how would the Bulgarians fare under Russian rule ? More like the Finns, with more or less automny or more like the Ukrainian, with heavy russification programs ?
Well any scenario where this happens will definitely have to be an uber-Russian wank considering that having Russian territory stretch all the way past the Danube and approaching Constantinople/Mediterranean ports will be a massive destabilizer to the Concert of Europe and will be violently resisted by whatever Great Powers exist.
Britain, France and Austria are the main Great Power obstacles, with Austria probably the most implacable one considering their security is quite tied to having no strong power holding the Danube. Greater Bulgaria will be fiendishly difficult as nobody wanted Russian power to extend all the way into the Mediterranean, especially not Britain and France.
A possible PoD could be:
1) The Habsburg Empire collapses in the 1848 revolutions, with Hungary getting independence.
2) Napoleon III comes to power in France and pursues a strong pro-Ottoman policy. Suez gets built, enhancing the region's importance to UK.
3) Without Austria to counterbalance France, Napoleon III embarks on more 'adventures' in Italy, maybe forms an axis with Prussia/German Confederation. UK-France relations go into deep freeze.
4) Mindful of Napoleon's legacy, Britain starts looking for allies in a potential war against France. Russia is a natural fit for that purpose.
5) Bulgarian uprising in Ottoman Empire, with France supporting the Ottomans. Russia uses pretext to invade Turkey, sweeping away weak Hungarian protests.
6) Nobody wants to see Russia gain a Mediterranean port but the UK sees benefit in driving a geographical wedge (of sorts) between France, Italy and Turkey. So they broker a deal where Russia gets Greater Bulgaria, but without a Mediterranean port.