Would be cool to have him in the background somewhere. Maybe not as a central focus since this is supposed to be a more simplified story that introduces new players to the franchise, but as a small nod to lore fans it could be great.
It would be nice to see him return, but I could see problems arising from having too many characters. Having too many characters weighs down the narrative, and prevents newer ones from being able to grow. I would be happy to see Jun in Infinite, but only if they keep the cast relatively small so that newer characters (like the Pilot) can be fleshed out.
At most have him be like a side mission thing, kind of like how Buck was in that pelican in New Alexandria in Reach, to where you can have it but can also ignore it. At least have Jun's voice be heard from a comlink or something in the background.