One thing I wouldn't mind seeing regarding stadiums is the ability to 'multi develop'. Basically, if you have the money, I don't see why you need to wait for one area to be upgraded before upgrading another part of the stadium (to compensate, maybe higher the number of days slightly to fully finish each one).

As an example, I think July Fourth recently mentioned that he has upgraded everything in the stadium without using any tokens to speed things up. Assuming he began upgrading from the moment he first signed up, I think he said it took about 15 MONTHS to upgrade everything (I'm level 6 and have only just Maxed Out the floodlights).