Reduce tuition to a sustainable fraction of median income to make an MIT education attainable to students without forcing them or their families to take on enormous debt.
I read that tuition for this academic year is $43k, more than 80% of US median income. That does not include room and board.
My freshman year 20 years ago, tuition was $19k, about 45% of US median income. Still not sustainable, but something that a family at 2x US median income would at least have a chance of managing.
20 years before that, in 1973, tuition was $3.5k, about 15% of US median income. For a family at median income, that would have been a very painful commitment, but one that would have been achievable.

I think alumni would actually be willing to donate if MIT published an aggressive endowment goal with the commitment to keep tuition at a level that is sustainable for a middle income family (like say 20% of US Median income)
I think if advertised and broadcast, it would also encourage higher participation and more generosity from students and families that are currently at MIT.
Such a move from a leader like MIT would put pressure on other universities to follow, which would solve a grave crisis in the US.