Suspect within' the Institute there were more infos about the mission Zimmer was into but they are not willing to share them with the Sole Survivor for some reason, since FO3 events weren't forgot neither was this character.
Been a while since i've finished my Institute ending playthrough, but iirc wasn't there something on terminals which completely suspended / deleted the cyborg programs and resources?
Imho many characters in the Institute should know about it, for example, how does Drs. Li made it to the Institute? Zimmer being the most reliable link for her to find and contact the Institute, whether she wanted to join with them.
And Ayo also should know enough , for the same reason that he is still "acting" director, so, if Zimmer was the SRB head his position is still vacant?
Or they know that he's anyway coming back?
He looked quite old in FO3, so that makes sense that he could've been simply died of old age, unless in the Institute they know that he can't age (somewhat like Kellogg maybe?).
Would be cool whether he'd return and allow the Institute to restablish the cyborg project under a new directorate, or just returning as a new villain, something like elder Elijah, opposing the Sole Survivor directorate role? unlikely, but well, would be nice.
Anything that deepens the Institute's lore would be nice, really.