While I'm sure that Arlington Center and Arlington Heights would love to have stations as alternatives to the 77 Bus, I think that because Lexington is likely to block anything, the Red Line will either have to terminate at the 77's terminal in Arlington Heights or any future Red Line will have to go out Route 2 (its possible, I suppose, you'd see branching).
I don't see Belmont letting you do a Red Line branch. Heck, its hard to even get the Mass Central converted to a bike path.
I think we're most likely to see a tunnel under Route 2, using cut-and-cover from Thorndike Field to the base of the big hill (too steep for a train, but fine for a long, deep, gently-graded tunnel that slowly climbs to the surface).
I'd put a highway-spanning park-and-ride garage at the Mass Highway Division site in/near/across Exit 59
http://binged.it/15AwqHx and run in the median of Rt 2 at the surface the rest of the way.
Next stop would Be Spring St (and, I'd hope, transit oriented development) and then I'd turn in any direction along 128 that would take me to a town that would permit a terminus (either Hanscom AFB, or south to Waltham, or North to a Rt 3 park-and-ride and a Burlington Mall terminus)