One thing that works for the Deer district is that it is downtown. People can walk not only to the Fiserv forum, but to the restaurants there. Why? Because there are enough condos, apartments, and campuses (MSOE and Marquette) nearby -- people live there. Am Fam field is still, sort of, in the middle of nowhere with the buffer of parking lots to help isolate it.
To have an entertainment district, there would need to be something similar to that at the Drexel Town Square in Oak Creek. You have to get people who will be living nearby. That and have a lot more restaurants, etc. that people will want to go to. Maybe there needs to be some more white collar businesses in the vicinity, too.
Imagine what could be if there was a real light rail system in Milwaukee. The Hop is, well, a bit of a joke. From what I recall, the Metro in Minneapolis gets people to Target Field, US Bank, etc. quickly and easily (I can certainly be wrong there). So... throwing spaghetti on the wall in a brainstorming capacity...
Take some of the area around Am Fam to create condos / apartments
Build something that will have well-established businesses move down there to get people into that district
Provide a better mass transit solution to get in and out of that area.
Put in a hotel or two and have events on a regular basis within Am Fam field in the offseason. There are plenty of meeting rooms, etc. where they can have larger events.
Create a covered walkway from the above items to Am Fam so people can walk from the hotels to the stadium in the winter.
Not sure if these are good ideas or if they are even feasible.