If they're really serious about this, they (Kraft plus Wynn) should invest in a new T line (the yellow line?) going from North Station up through Charlestown kind of along rte 99 and have some much-needed stops in Charlestown, for the stadium, then casino, and continue on up through Everett, into Malden (Granada Highlands area), and ending somewhere in Saugus near rte 1. That is a HUGE community area that desperately needs something other than painfully slow buses that take forever to get into downtown. That would alleviate a tremendous amount of the traffic that currently drives to the casino, new developments and an entertainment complex and would certainly entice people who are meh on soccer but love live sports and hate driving around there (people like me and most of my friends).
This is the same city/metro that took three decades (or four, or longer, depending on how you want to look at it) to add six stops -- six easily implemented stops (pre-existing track space/path) to the Green Line passing through the most densely populated region in the Northeast (Somerville) that needed additional T service forever.
The notion that anyone -- even extremely "well heeled" folks/organizations such as Kraft and Wynn -- will snap their fingers, open their wallets, and just like that we'll get a whole new T line is... unlikely.
I agree it'd be great to have and would get tons of use by an as-yet underserved community, but for now I'd settle for trains running at top speed without catching fire or dragging anyone to their death and for the fullfillment of the disastrous CRRC red/orange line orders.