You're right to be skeptical, because Romania was perfectly happy with expanding beyond territories populated by Romanians or traditionally Romanian. See for example the takeover of Dobruja, especially Southern Dobruja, the attempt to expand up to the Tisza river after WWI or the conquest of Transnistria during WWII. All Balkan states have made declarations of only wanting what was theirs, but it has rarely been true in practice.
Dobruja was not annexed directly to the Romanian state until 1908 or 1910, can't remember exactly, until then it had been a sort of colony. The reason: the Turks and the Bulgarian formed an overwhelming majority. Regarding the Quadrilateral (Southern Dobruja) it was merely grabbed as a token and to show the Bulgarians "who's the boss". Practically, Romania was trying to prove to the other Balkan states that the she was the regional power and if needed it could intervene to keep in check any potential rival. Even considering that, both Northern and Southern Dobruja had been a part of Wallachia during Mircea the Elder's reign in the 1390s. The Tisza border had been claimed before by the Romanians (see Mihai Eminescu's poem "Doina". Its first verse is "De la Nistru pân la Tisa" which roughly translates to "From the Dniester to Tisza", the full extent of the irredentist claims of the Romanian nationalists. Transnistria was not "happily annexed" by Romania. It was given as a tradeoff by Hitler so the Romanians would shut up about the Second Vienna Award. The Romanian state never even annexed the territory, it was virtually held under occupation until the Soviets returned.
With a POD in the early 1800's we have
So, taking all this into account, IMHO Romania was already GREATLY wanked in OTL if we compare the situation in 1800 with that one after the WWI.
Pretty much. To have an even bigger wank we need to go before, as I said during Michael's reign, with a POD there we might even see a Romanian Balkan Empire (which, truth be said, is gonna be really hard to maintain considering the Habsburgs and Russia are getting stronger with every decade, but not impossible)
This wouldn't mean a large influx of other nations into the Principalities? First the Greek and South Slavics fleeing the "Turkish yoke" and later the Germans and Hungarians from the HRE and Austrian Dominions, making the Principalities a melting pot like the Banat or Transylvania? This wouldn't be bad to a national identity after all?
That's true, but I doubt it will be a large enough influx to destabilize the ethnic balance. And even then, the Romanians would still be the second ethnicity in the Empire after the Germans. It would also help westernize the Romanian society earlier.