Well, Wallachia and Moldova were colonized with Romanians from Transylvania who brought a even larger Romanian population in those lands.
Maybe the anti-Romanian and anti-Orthodox laws are becoming more aggressive, and a larger number of Romanians are emigrating to Wallachia and Moldova after the Mongol invasion.
But the most practical is if Michael the Brave manages to unite the three states permanently, and his successors manage to create a powerful and stable Romania, thus helping a lot to increase the Romanian population, and in TTL Romania reaches a population of 60 million or so like OTL United Kingdom.
Is not necessary this scenario, but the important thing is that Romania to not become a vassal state of the Ottoman Empire, and then under Russian occupation plus the destruction of WW1.
If Romania mananged to became early independent ( the revolution of Tudor Vladimirescu from 1821, for example) and managed to industrialize quickly in the 19th century with a strong leadership plus to avoid the destruction of WW1 on the Romanian soil, the things will go well for the population and for Romania in general.