You have to look at what is the best way to maximize revenue, not maximize attendance.I’d rather set a goal of selling out the current stadium before even thinking about expanding it.
If the stadium were to be expanded what would have to happen in order to regularly fill it. Expensive promotions and cheaper tickets would be big parts but both are self-defeating. The promotion is costly and cheaper tickets devalues the other seats you are selling.
Better situation is to have your current stadium at or near capacity, at least for multiple key games each year. Then you don't have to discount prices.
More importantly you create a situation where people believe that to make sure they get tickets for the games they want they might as well buy season tickets. Along with those season tickets for better seating you have additional rights fees or seat donations.
My plan would be to increase the number and quality of the premium seating and increase the average revenue per seat, not just the number sold.