Must be domed and air conditioned for the brutal summers. So keep it on the small side (reduce A/C costs) and make the real money on corporate boxes and the cable TV contract.
Fenway is only 34m seats. Its a 102 year old dump but has on average one of the highest ticket prices in MLB. NESN is where the Red Sox make all the money. You do not make a lot of money off the fans in the upper deck of the stadium.
They should build this place with a roof that can open and close depending on the weather. Vegas is a tourism city. I could see flying there for a weekend series. Taking in a couple games on a weekend.
Keep in mind that stadiums like Tampa make more money when the Yankees or Red Sox are in town playing the Rays. Especially early in the season and in September IF their respective team is making a run.