If he loses to Fulton his resume right now does not stack up to being an ATG. It's not his fault HOF ducked him and he doesn't have a stacked resume but to have his level of opposition and to be an ATG he needs years of dominance like Wlad to be considered an ATG which he could still do easily.
Inoue is right now, without a doubt, a HOF fighter. He needs to do more to be considered an ATG in my eyes, but he's indeed been Monstrous so far in working his way up. He looks like the real deal to me and is possibly the greatest P4P fighter on the planet. Your scenario, if seen to fruition, may make it impossible for Naoya not to be considered an ATG. I think he has what it takes to accomplish it, but he must not waste a single opportunity to fight the very best in every single fight and if he takes a loss or two along the way, he must rematch them as quickly as possible and must win.
HOF fighter not ATG yet he hasn't beat any P4P fighters and his best win is over an aging Donaire. Still needs a signature win or two for me Fulton is a step in the right direction.
My opinion may not be popular but I personally think Inoue is a borderline HOFer (but can punch a ticket to Canastota with an impressive victory against Fulton as Fulton is a P4P type fighter (top 10 to 20) that can raise Inoue's profile). I think ATG is a very difficult status to achieve in today's boxing landscape as talent tends to be very dilute. Modern day fighters I believe that achieved ATG status include Manny Pacquiao`, Floyd Mayweather`, Bernard Hopkins`, Roy Jones`, Juan Manuel Marquez, Chocolatito, Canelo (possibly if you're able to ignore the blemishes from the first 2 Triple G fights), Juan Francisco Estrada, Usyk, etc because majority of these fighters have fought multiple HOFers and P4P-type fighters. Nowadays, it's rare to see 2 P4P fighters face each other. But now with the increasing amount of undisputed fights being made and more elite fighters fighting one another, I think the amount of ATGs may increase going into the horizon. To add, due to Inoue fighting a lot of high level opposition compared to his P4P contemporaries today, Inoue has a very good trajectory for his legacy.
I believe Inoue's achievements make him an all-time great but if you were to rank the ATG's he would be low on the list. In order to move up much more the Monster needs a couple of quality wins over in-their-prime top fighters to add to his achievements.
I think he is. When you have a great series against another great you don't have to do a lot beyond that. He can climb a lot higher.