The only proposed extension of the TT's is 71 to Newton Corner where it would tie into a bunch more routes and offer better Waltham and Pike express bus coverage. It's rated in the 2003 PMT from Boston MPO:
http://www.bostonmpo.org/bostonmpo/pmt-old/PMT-5.pdf. $1.5M to build (price includes 1 extra bus, so infrastructure alone with the current fleet numbers that's more like $600K-$750K). Adds 800 new 71 riders and +600 riders taking no current transit.
The 71 and Watertown Carhouse are still powered by the old Green Line A-branch underground power cable, which is a FY2016 unfunded line item on the budget for replacement for state-of-good-repair and bumping the juice a little on the Watertown end. That's all the infrastructure needed to handle this extension. So it boils down to running the 1/2 mile of extra overhead down Galen to loop around the Pike rotary, and plugging it into the same spots where the underground cable powered the Green Line overhead until it was shut off in '94.
I think they should wait on this one until there's commuter rail stations added at Newton Corner and Allston with some high-frequency Worcester Line service in the inner 'burbs. Then this is a really high-value connection where CR at Fairmount-like headways can replace the Pike express buses and the 71 gets kicked up a notch as a very big-deal connecting route.
Other one I could see is simply extending the 77A wires to Alewife Brook Pkwy. and the proposed Alewife busways. Another small addition that doesn't require much upgrade whatsoever to the central power draw, and makes it so both the 77A and 79 each bolster half of the 77 coverage to the midpoint. And...if they bought more Silver Line dual-modes you could run the articulated buses under the wires on the full 77, turn out at the Route 16 stop, and power-switch to/from diesel in Arlington. Cambridge would definitely go for that. Think in the interim though there's a lot more they can do to improve the 77-proper before they need this. If the Green Line were extended someday from Union to Porter, then that transfer and downtown rapid-transit bypass would flush enough new 77 ridership (esp. out of Arlington) to merit this TT flesh-out handily. But not before then. I'd rank the officially-proposed 71/Newton Corner run a lot higher.