The jug is a real water jug, from Minnesota, purchased by Michigan. It's inherently connected to both states. The Axe, is a link to the shared legend of Paul Bunyan, important to both Minnesota and Wisconsin. (Paul himself is Minnesotan. He only went to Wisconsin to visit the Dells) The Pig was originally an Iowa Pig, won by the Gophers and named for the Minnesota Governor and the Iowa farm the pig came from.
None of these are conventional trophies. The Bell is pretty much a generic trophy. It's bland. You would have no idea that it is for something special. One thing that galls me about it is that it uses the Victory Bell. While the Victory Bell is an important icon to all Americans, it's especially an icon for Pennsylvania. So, we have this generic trophy, which celebrates a cultural icon of one state, but not of the other.
I mean no disrespect to Penn State. Beating Penn State is great, but that's just from beating a respected team. Winning the Bell just doesn't add that much to it. With the Jug, the Axe and the Pig, that makes it big no matter what the records are. Nebraska and Minnesota were once rivals. I think a 5th trophy would be pretty absurd though. If we did have one, it shouldn't be generic, it should be something important to both states.