I had a blurb last April about the plan to separate W&OD trail users on foot and those on bike. The day before that published, the TPB approved $274,250 to do just that on a section of trail from N West Street to Great Falls Street in the City of Falls Church (along with some other projects that I'll write about later).
Project will replace .5 mile of 10 foot wide shared use trail with 11 foot wide bicycling trail and
8 foot wide pedestrian trail separated by a 2 foot wide median.
Design and Engineering work will go on in 2019 and construction in 2020.
The Falls Church project wasn't fully funded, but when it is it will similarly widen the rest of a 1.2 mile section and it will replace six substandard curb ramps and a narrow trail bridge over Four Mile Run.
The idea dates back to at least 2014, which is where the image above and the maps of possibly widened trail areas below come from.
Speaking of the W&OD Trail, some trail adjacent development could occur in Reston if the Hidden Creek Country Club is developed. It's hard to tell from the images at this link, and I can't find any information on it, but one would hope that whatever happens, a trail connection is built through the development and the trail gets a trail-facing building or greenspace, instead of a back wall.