While the Blue Line extension to Charles/MGH would be a good thing to build, I don't think it's the low hanging fruit in reducing dwell time at Park St and Downtown Crossing.
http://www.archboston.com/community/showthread.php?p=183716 has F-Line to Dudley proposing ``a second Red-level Park St. egress/headhouse onto the Common from the emergency exit end of the platform to relieve the overstuffed stairs and worsening dwell times.''
There's the existing Park St underpass that connects the northbound Green Line platforms to the southbound Green Line platforms. The west end of that underpass is the elevator machine room for the southbound Green line platform elevator; if you relocate that elevator machine room and maybe enlarge the elevator in the process, the door to that elevator machine room could turn into the entrance to a tunnel going under the westernmost Green Line track, and then that tunnel could go diagonally to the midpoint of the Ashmont / Braintree bound side platform. You probably want a tunnel 20' - 30' wide for that, and you probably need to widen the staircase connecting that underpass to the southbound Green Line platform, but something along those lines could probably help to convince some of the passengers transferring between Red and Green at Park to use something other than the two easternmost cars of the Red Line trains.
There are also lots of opportunities to improve bus service to divert some trips away from Park / Downtown Crossing, like extending 28 to Kendall, 23 to Central Sq in Cambridge, getting rid of the 1's crazy indirect route to Dudley once 23 does Central to Dudley directly, and then you can have 1 be Central to UMass-Boston, which means that Green Line riders from west of Copley who want to get to UMass-Boston can just transfer to the 1 at Hynes / Prudential and stay out of Park.
32 could be extended north along South St, Centre St, and South Huntington, and continue from there to Harvard and/or Central and/or Kendall.
More generally, West Station should get a busway that runs north-south to connect it to Harvard, so that you can have routes that originate at Harvard and continue to West Station, and from there one route could continue along the B branch to Boston College, another route could head down Babcock to Coolidge Corner and out to Reservoir, a third route could head down Babcock and continue by mirroring parts of 66/39/32 as alluded to above, a fourth route could be a revised 22, running from Harvard to Coolidge and then follow a bit of 66, a bit of 39, a bit of 14, and then most of the current day 22, terminating at Ashmont, a fifth route could go from Harvard to Coolidge, run along Longwood Ave, then St Alphonsus St, then Tremont St, then continue along 15 to Kane Sq and continue from there to Savin Hill Station. It's important to make sure that West Station is designed so that these routes can proceed efficiently through West Station; the West Station plans I've seen so far seem to envision something that will work about as badly as bus circulation at Dudley has been working in recent memory.
http://www.archboston.com/community/showthread.php?t=4473&page=18 has some proposals for Wonderland / Chelsea to greater Cambridge bus service.
http://www.archboston.com/community/showthread.php?t=3664&page=144 suggests extending 110 to Davis.
We could probably be making some incremental progress on many of these things for relatively small money while we wait for funding to appear for the Blue Line extension.