I would wish the newborn child after Diana would live.
If season 5 were to appear, that child would be a big part in the future of Grimm. He and Diana could have a part in a destiny that'll decide the fate of all of humanity & Wesen. Including the Grimms.
Nick could be unstable for half of the season when he loses Juliette and his mother. For this, he gets his revenge on the Royals and the Wesen council.
Adalind, Monroe, Rosalee and the rest of the scooby gang will feel with Nick feeling despair. The only hope to save him is the newborn child.
I don't want Sean to die! If someone replaces him as the new captain, who knows what will happen. The new captain might reveal all of the Wesen and have the police hunt them all down. Without Sean, there's no way to protect Nick and bring him back.