Okay... What do we have here?
Premise: Obscure POD in a little unimportant country.
Results: Unknown, asked for.
Choices:
1. Hard AH. Investigate all possible effects, all over the World. The Butterfly Effect rules supreme. We may not have a WW1 or it may happen earlier or later with vastly different technology and results, to say nothing about the possible alliances. By the time of the OTL destruction of Romania, the World may be unrecognizable. Innumerable variants here, including authorial fiat.
2. Soft AH. Except what is clearly changed by the POD and its direct consequences (mostly inside Romania, at least in the first years), the history of the World remains mostly the same. We have one or more Balkan Wars in the 1910's and at least one World War shortly afterwards with more or less the same participants. Romania may fare better or worse (due to my massive and obvious bias, it will fare better).
3. BKTL AH (Butterfly Killer TimeLine). Everything stays the same (except what we want to change) until some time when Romania is ready to do whatever we planned for it. Well... Whatever.
The following example could be characterized as Soft AH.
8 September 1870 (POD):
Domnitor Carol I of Romania and his wife
Doamna Elisabeta have twins: a son,
Carol, and a daughter,
Maria.
1874: A scarlet fever epidemic kills many children in Bucharest. Prince Carol and Princess Maria do not catch the disease (in OTL, Maria died). The relation between Domnitor Carol and Doamna Elisabeta remains good, the Domnitor keeps working in full capacity for the betterment of his country and the political situation remains stable, aided by the presence of a clear and undisputed successor of the throne.
1877-1878: The Russo-Turkish War (Romanian War of Independence) occurs on schedule. The Romanian Army crosses the Danube and helps the Russians defeat the Ottomans as in OTL. A somehow better performance of the Romanian Army (than in OTL) has two small but important consequences:
- The Vidin Area (liberated by the Romanians as in OTL and with a Romanian plurality as in OTL) is annexed to Romania;
- Southern Bessarabia is ceded to the Russian Empire (in exchange with Northern Dobruja, as in OTL), but the Romanians manage to talk the Russians into recognizing some measure of National Rights for the Bessarabian Romanians (while the Russians would not fully respect the treaty, it was better than nothing).
The Independence of Romania and the annexation of Northern Dobruja happen as per OTL.
1881: Independent Romania becomes a Kingdom (as in OTL). Domnitor Carol becomes King Carol I of the Romanians, Doamna Elisabeta is Queen Elisabeta and Prince Carol is the Crown Prince.
1880's, 1890's and 1900's: Romania's demographic, economic and military progress is (a little) greater than in OTL. Young Prince Carol follows a military career and the Romanian Army is taken good care of.
1910: The Balkan War. The Balkan League (Romania, Bulgaria, Serbia, Montenegro and Greece) defeats the Ottoman Empire and captures all of Ottoman Europe except Constantinople and its environs.
1911: The victorious powers manage to peacefully divide the spoils averting a Second Balkan War among themselves. Serbia unites with Montenegro and gets the Sanjak, Kosovo, northern Albania and NW Macedonia. Greece gets all the Epirus, southern Macedonia with Salonika and the Chalcidic Peninsula. Bulgaria gets NE Macedonia and all Thrace except a sliver of land near the Sea of Marmara and the Turkish Straits. Romania is compensated with Silistra and the coast up to Balcic (about one third of OTL Cadrilater / Southern Dobruja). Central Albania becomes an Austro-Hungarian Protectorate.
Voluntary population exchanges are encouraged by all Balkan League Powers. Numerous Aromanians are settled in Dobruja and Vidin.
1912: Start of World War One. Austria-Hungary could not tolerate any more Serbian and Romanian encroachment and provocation (Romania was not a Central Powers Ally). A cascade of alliances and declarations of war delimitates the two warring blocks: The Entente (UK, France, Belgium, Russia, Romania, Serbia, Bulgaria, Greece, Japan) vs. the Central Powers (Austria-Hungary, Germany, the Ottoman Empire, Italy).
1912-1918: [Most of the details of the war are not clear and are not supposed to be for such a short limited sketch.]
In the West, the war is mostly static as in OTL. In the East, A-H and Germany invade and occupy most of Serbia and Romania. The Romanian Monarch and Government retreats to Dobruja. Russia collapses and is destroyed. Germany occupies all territory up to the Don. The Entente kicks the Ottomans out of the War. Constantinople and most of Arabia are occupied.
The war ends with general exhaustion. A-H collapses and is dismantled. From Paris to Moscow, Europe is engulfed in various Communist or Nationalist Revolutions.
1913: King Carol I dies. Crown Prince Carol becomes King Carol II.
1919-1921: Paris Peace Treaties. [A map would be nice, but since this is not a proper TL, I don't know...]
Romania gets everything from the Tissa to the Bug, minority treaties notwithstanding.
1960: King Carol II dies 90 years old. His grandson succeeds him as King Carol IV.
What had happened in the meantime? I don't know. Throw in some (aborted) Communist rebellion, a short war or two with rump Hungary, maybe with the Ukraine as well, issues with the minorities, the obligatory 20th century antisemitism and even a Second World War if you feel like it (although the lack of a revanchist Germany should preclude that).
2017: Romania is a multiethnic country (about 60% Romanians), powerful, stable and prosperous. King Carol V had just acceeded to the Throne.
As I said, my two active TLs and numerous other ideas do not allow me to pursue this at this moment. I would like to read something like this though (hint, hint

).