Oh, definitely not all of Transylvania. Making Transylvania 100% Romanian would be as silly as having it be 100% Hungarian

Currently Transylvania has 5 provinces. I think the best option would be to add 1 Romanian province and 1 or even 2 German ones. The Romanian one should come from splitting up current Maramoros into Maramoros (Romanian majority for a very long time; it was a mountainous region where the Romanian/Vlach shepherds lived for a long time and actually started many of their long migrations that brought them even to far away places such as
Moravian Wallachia in Bohemia of all places!

) and
Zakarpathia (Ruthenian). The Ruthenian one probably needs another name.
For the German ones, a Saxon one could come from splitting Hunyad in two, making a new province around Hermanstad (Sibiu). Similarly for Bihar: split it up adding a Swabian province around Sathmarer.
Something like this, I'm sure there are modders who can explain it better than me.
Having Slovaks in Northern Hungary wouldn't hurt either, but I know I'm pushing it