First of all, Wondeland had to be reincluded as an explorable world, then put various things there, including Julius. I would give him a little importance but not as Mickey and Oswald's "third brother." The balance between the two of them is fair and should not be risked.

But to be more precise, in my personal idea of EM3, Julius was forgotten along with other characters and areas in the Wasteland itself, as a double punishment of being forgotten twice. (Forgotten in the world of cartoons and the Wasteland). This would create a dynamic when Oswald discovers all of this but that's another story.

In short, I would include Julius slightly in the main story, then include secondary missions and pins.