Its antagonistic Shanks. He does not need to be evil.
Its also pretty obviously the case since Shanks goal is currently the same as Luffys. One way or another Luffy and Shanks will have to fight. Either each other or someone else.
The theory was about Shanks being some kind of final villain though, which he isn't going to be. he can't even be a credible villian since he isn't it; BB is built for that role.
Sure, there might be some scuffle with Shanks since Luffy kind of said it back in PH, but it's clear BB is going to kill him or anyway be his main fight.
Wishful thinking. It would shit on Ace really hard if someone else with his fruit was actually capable and not a bum.
Also the only redeeming grace of Sabo is that he isn't actually an Ace 2.0: he has a different life objective and a completely different plot, all centered on the WG; he has declared war on them, seen Imu, seen the Gorosei, seen Cobra killed, seen Lulusia, he is the bloody second in command of the revolutionary Army and last but not least was a DC the cause of his amnesia and that gave him a scar (and in OP scars are symbolic).
That is his plot and it's what make Sabo a character of his own.