I don’t consider Inter Milan returning for Onana as a wise decision after selling him out to Manchester United, not even that he has impressed with Manchester United in his stay so far so I don’t see reasons to want Onana back as number one goalkeeper choice for Inter Milan next season there are other choices in the market, Martinez from Aston Villa is also available this summer.
Inter was not a dumb team. They would rather get Martinez from Villa instead of Onana. They successfully scammed MU with Onana's transfer. Why do they need him back? The better to let him stay in MU while they approach a new, cheaper, and capable goalie.
Bringing back Onana to Inter Milan is a very bad idea for them. If that will go for Martínez, although they might think after they bring him back he will be able to do something better for them, I don’t think that will happen. If they really want to get a better goalie, I don’t think Onana will be a better option. But let’s see if they will take him or not. I don’t think they will end up making that decision, to be honest.