Taking the issue of concessions and restrooms separately, I think there is a fairly simple solution for the first but not so much the other. I was at the Cincinnati game (as I am for all home games) and the concession situation was disgraceful. I had no issues with the restroom line - and I'm sure I went at least twice because I always do.
As for concessions, switching from fountain drinks to exclusively bottled beverages would speed up lines significantly, and reduce the number of workers required. It would also allow for the addition of small kiosks offering only drinks and pre-packaged snacks, adding additional concession capacity with very minimal investment.
Restrooms are another issue entirely. In addition to materials and labor to build/install more toilets and enclosures ( assuming you add actual new restrooms), you also have to consider the need to add water and sewer capacity - which would likely require millions of $ for those areas on the east and west sides that are original to the stadium. Furthermore, if you touch those existing restrooms or utility pipes, you have to upgrade everything there to present code - plumbing, electrical, ventilation, doors, ADA requirements, etc. Those are complex and expensive improvements. Unfortunately, this ain't like remodeling or adding a bathroom to your house.