Union of Moldavia and Wallachia!
It is available to Moldavia and Wallachia. It requires that both nations are not puppets and have invented State and Government. (They both start as puppets of The Ottoman Empire). Romania must also not exist. When one of the two countries take the decision, it will change the country to Romania and give 10 prestige.
The player has to watch out, if one tries to enact this decision. It is not likely that only one of the two nations can enact the decision while the other one is not able to so. Only very early in the game where, State and Government might not be invented by both sides. So it will become a race to press the button.