Grossdeutschland would, upon it's inception, pretty much automatically be the most powerful country on the planet. It'd have the economic, industrial, and military might of the German Empire, which was already enormous and already arguably the most powerful upon it's foundation IOTL. Then, throw on the richest and most industrialized parts of the Austrian empire, an extra roughly 15 million people, enormous reserves of coal and gold, and a presence in the Mediterranean. The European powers would never let this happen- the only ones that would have anything to gain would potentially be Russia and the Ottoman Empire, who, without Austrian tampering in the Balkans, would be more free to mess around there. Other than them, basically every single European power would try to strike it down. Granted, that would likely just amount to France, the United Kingdom, and... actually, no, probably just them. I imagine that they would grant the northern Italian portion of the Austro-Hungarian empire (excluding South Tirol) to Sardinia-Piedmont, create a Polish state in Austrian Silesia, leave the Kingdom of Hungary as is, just independent, and let B-H do as it will, likely joining Serbia. If the United Kingdom and France tried to attack and put down this country, well... it'd fail. It would basically be much like WW1 in 1914, except no eastern front, and with Germany being far, far more powerful. Germany would almost certainly win, take Alsace-Lorraine and a large sum of money, and perhaps some colonial concessions, sewing the seeds for future conflict as the UK and France desperately attempt to battle rising German hegemony.