Not saying this is for definite, but maybe if Naruto and Sasuke were to have their Rasengan and Chidori clash again like they did during the rescue Sasuke arc, the amount of passion they put into those attacks may have given us an early glimpse at the sealing technique they are able to perform together once their full potential was unlocked.
I'm probably 99% wrong but it could be a fun idea as they would ultimately die together to save the world.
Just a reminder of what it began to look like below.
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