I think Coor's should just allow a 1-3 story building, such as a Target (as many have brought up before). This or another retail company could work out a situation that ends up benefitting both sides. For example Coors get an annual fee for the space, but where a Target, or something of the sort, would be allowed to place some kind of moderate, yet visible signage that could be seen on TV. Companies would pay to see their name, and location, every time a homer is pelted at Coor's field if you ask me.
Make it cute for all I care and include the Rockies/baseball in the sign. Especially with the Target logo. It goes hand in hand with baseball. "Homer here for mile high deals!" Lame, but seems logical enough to me.
Everybody wins, and we get our downtown Target.