VDOT, in case you haven't heard already, is planning to expand, improve and modify I-66 in northern Virginia. And part of that improvement includes building a trail along I-66 from Dunn Loring, where it would connect to the W&OD, to Haymarket. Technically, I suppose, this wouldn't be an extension of the Custis Trail, since that ends a few miles away in Bon Air Park, but I've been calling it that due to the similarities (But, I've changed the category for the Custis Trail to Custis/I-66 Trail because of this).
Below are images of the easternmost panel, but there are is one other for Fairfax and two more for Prince William, at the links. Wedging a trail through a built up area, next to an Interstate, like this is not easy and it makes for some weird detours, difficult crossings and odd routing. One debate that will change the feel of the trail is whether it will be inside the sound wall or outside of it.
East end of the trail from the W&OD Trail and Dunn Loring Metro station.
The trail in the area of the Vienna Metro Station
The trail from the City of Fairfax to Fairfax Corner
The biggest surprise to me is that it doesn't explicitly connect to the Cross County Trail at the Vienna Metro Station, where the terminus of the XCT is only about a block away from this trail, but hopefully that can be corrected later.