Does he deserve to end his career at Wrestlemania?
He had a great run in WCW that many remember him for, and he also had a WWE run which ended at Wrestlemania 20. If his career ended at Wrestlemania I sure hope it would be better than THAT match. He does deserve to go out better than that in my opinion.
Who should end his Career?
It is difficult to say. He would obviously want to win the match. Who on the roster could put on a decent feud with Goldberg and not be damaged by a loss? That is the better question. Orton could put on a good feud with him and bring back his Legend Killer persona if even just for that angle. I'd want Orton to win though and Goldberg might not agree to that.
and would it Overshadow Rock/Cena if it happened at WM28?
Not a chance. The one and only way that ANYTHING could EVER overshadow Cena VS The Rock is if Taker's streak ended because it takes a huge shocking spoiler to be a bigger deal than a dream match between icons of two eras, and we all know the odds of the streak ending are next to nothing.