Not saying that the boxers he has faced and the others on that division aren't that great.
But if he's the type of boxer that's up for challenges and he's proving something to himself, that's what he must do and move up until he proves to everyone how the monster of him managed to get up to several divisions and belts.
Definitely, it doesn't make sense for him to stay at this division since he clean up it already. Winning WBSS against Donaire, and then in his last fight he knock him out. So if he beat Butler by any shape, then he should go up and Fulton and Murodjon. This two has divided the belt amongst them, so clearly that will be the target of Inoue and this could be his 4th weight class with a belt.
If he just keeps on dominating his current weight level, there's really a need for him to do either going up or lower down. Just like what Manny Pacquiao did.
But this time, it's not a Filipino fighter that's doing it but a strong japanese boxer.
He loves challenges so that is the best options for him, chase those champions in the higher weight class.
One thing that goes for him is that he is being managed by Bob Arum, who already has the blue print on how to go up in weight and face the champions and claim their belt and then go chase the next. But probably I'm seeing his getting to be, and not sure about it, be the first Japanese to move at least 4-5 division south and win a belt. So far Canelo has 4 and chasing a 5th to be the first Mexican to do so. Inoue can do that as well to be the first Japanese.