Hi,
What about the large open field near the baseball field where school students play soccer in the evenings?
(Street view -
https://maps.google.com/?ll=42.3764...VAMwMJjTA32vVPv4dQ&cbp=12,47.35,,0,-2.88&z=15 )
The side with the small kiddies park is also fairly devoid of large tress and once down a good 20-30 feet might be pretty well devoid of much roots as well. Don't forget the Red Line is already under far eastern most portions of the Common.
(Street view --
https://maps.google.com/?ll=42.3778...oQWJ0yMKO56KvWmLjChuwg&cbp=12,166.34,,0,-2.55 )
Between Waterhouse St. and Mass. Ave. there's a large clearing where the large surface grates for the Red Line are situated that's a perfect spot for a ramp downward.
Street view -
https://maps.google.com/?ll=42.3796...=7w5tmRvPbHAQ23fn9Ol3Xg&cbp=12,202.48,,0,2.14
A few years back the large grates near Mass. Ave. and Garfield were better-integrated into a new housing development. Something similar could be done with parking structure.
(Street view -
https://maps.google.com/?ll=42.3836...=ZFakUMZaBeLEPdy7P3hH1A&cbp=12,102.35,,0,1.32 )
Like Alewife station, I bet it would often be full between Harvard University, local businesses, residents, and MBTA riders. The increase in new buildings around Harvard is going to make the parking situation tougher and tougher.