The Germany international has hit the ground running at City after joining from BVB recovering from knee surgery.
Gundogan told BILD: "If it would happen in the next year or two, I would not be happy about it. For it would probably mean that I'd failed or had not developed.
"If I'm 29 or 30, I do not know if (Dortmund) will still want me.
"But basically, I do not exclude the possibility, but now I have other big goals."