Athalaric was the grandson of Ostrogothic king Theodoric the Great. His mother and regent Amalasuintha, tried unsuccessfully to give him a Roman education. She was also in good terms with the Eastern Roman Empire, allowing their fleet to use the harbours of Sicily during the Vandalic war.

Let us suppose that around 528 A.D, Amalasuintha persuades Justinian to adopt Athalaric or take him under his protection.. The Vandalic war goes as in OTL..

What happens next? An Ostrogothic nobility revolt? What if the alliance stabilises?
Little changes. The Ostrogoths are not going to accept someone with overt Constantinopolitan influence as their ruler. Justinian invaded using Amalasuntha as a pretext. If Athalaric somehow survives to witness the subjugation of Italy, what's to stop Justinian from getting rid of him in an accident and blaming it on the Ostrogoths? You need someone who could balance the interests of the Roman senatorial aristocracy who may or may not favor Constantinople and the Goths, like Theodoric did for the majority of his reign.