The one major corridor I could see is VA-3 on the Northern Neck. That was proposed for 4-laneing in the 1980s between Culpeper and White Stone. The sections between US-29 at Culpeper and US-301 were 4-laned. Between US-301 and White Stone several short segments were 4-laned. The rest should be 4-laned IMO.
The corridor between US-301 and White Stone is 70 miles long, with about 16 miles already four-lane. That leaves 54 miles of two-lane roadway that would mostly be dualized, plus at least 5, possibly more, densely developed areas / towns that would need to be constructed on new location.
Could see it under Virginia's original arterial system, though it's questionable if it'd be a priority in today's funding world. The corridor has mostly 2,000 - 5,000 AADT in rural areas
(with some areas having 2,000 AADT in the middle of the corridor, it indicates there's very little long-distance traffic), with up to 8,000 AADT near & in between a couple of the towns / developed areas. Assuming $20 million per mile, around $1.1 billion.
Some other corridors that come to mind that could've / should've been 4-laned are US-258 between Smithfield to Franklin and VA-10 between Smithfield to Suffolk.
VA-10
(between US-258 to US-58 Bypass)- 13 miles long total
- 4-lane: 7 miles
- 2-lane: 6 miles
- AADT: 12,000 south of Smithfield on 2-lane segments
US-258
(between VA-10 to US-258 Business / US-58 Business)- 31 miles long
- 4-lane: 2 miles
- 2-lane: 29 miles
- AADT: 3,000 - 5,000 AADT west of Smithfield on 2-lane segments; 10,000 outside Smithfield, 19,000 on Smithfield Bypass
A study was completed in ~1994 that evaluated widening US-258 between Smithfield and Franklin to 4-lanes, which mostly involved retaining the existing roadway and constructing a parallel roadway, and featured a bypass of Windsor to the north with a partial cloverleaf interchange at US-460.
I'm not aware of any study that has been conducted on VA-10 to close the "gap" between the 4-lane segments. Reasonably, the 6-mile stretch could be partially dualized, though some of it may need new location, and definitely a new location bypass of Chuckatuck.
My thoughts - US-258 to Franklin would be ideal, but like I've mentioned before, with today's funding system, probably unlikely ever to be a priority. As for VA-10, it's definitely needed, with the growing traffic between the two cities, all the development occurring in Suffolk and in southern Smithfield, and also the fact the corridor
(VA-10 & US-58) acts as a "bypass" of the congested US-17 segment when linking to areas south of I-264.