Partial list for the Illinois portion of Chicagoland:
* US 12: End of the four lane section west of Fox Lake to the end of the freeway in Wisconsin
* US 20: West end of the Elgin Bypass to at least Plank Road.
* IL 31: Fill in the gaps between IL 72 and IL 120
* US 45: End of the four lane near Washington Street to at least IL 173. Also the diagonal section between IL 83 and IL 21
* IL 47: End of the four lanes at the north side of Huntley to Charles Road on the north side of Woodstock. Also the end of the four lane section north of Yorkville to IL 56
* IL 58: End of the four lanes at Barrington Road to at least IL 59.
* IL 59: End of the four lanes north of IL 72 to at least US 14 in Barrington, if not US 12
* IL 60/IL 83: The whole joint stretch
* IL 62: Fill the gap from IL 25 to IL 68
* IL 83: South of IL 132 to IL 137
* IL 120: Probably all of it in Lake County, but at the least fill the gap between US 45 and the end of the four lane west of Hunt Club Road
* IL 131: At least IL 173 to the Wisconsin border
* IL 173: IL 59 to at least I-94, if not all the way to IL 131
* Barrington Road: Fill the gap from Mudhank Road to IL 62
* Palatine Road: Between IL 62 and US 14
Yeah, there's a lot to list. I'm going to add some from the Chicago Southland:
-US 30: all remaining segments east of IL 47 (including the overlap), especially between Briggs and I-80, and the segment from Plainfield to Oswego.
-IL 7: Entire route. Some is being done right now, but the Lockport bridge really needs an expansion next.
-IL 83: Cal-Sag portion.
-Laraway Road: Harlem to IL 53.
I'd add -
All the gaps in the WiKaDuKe Route from US 6 in Minooka to the 4-lane section of Eola Road. There are some portions of 4-lane in Minooka and Joliet, but the route would function a lot better if the gaps and discontinuities were addressed.
IL 71 from IL 47 in Yorkville to Orchard Road in Oswego
US 6 from the 4-lane section in Channahon to Ridge Road in Minooka
US 6 in Morris from IL 47 to Saratoga Road
I'd also fill in the gap on IL 47 from Caton Farm Road south of Yorkville to IL 71 in Yorkville, but only after the Yorkville to Sugar Grove portion was completed. (the section to the south is currently under construction to 4-lane from Morris north of I-80 to Caton Farm).
At some point, IL 31 and US 34 between Orchard Road in Oswego and BNSF overhead in Montgomery should be expanded; it was under study in the 90s and the idea abandoned in the hope that the new 4-lane Orchard Road would relieve it. It did a little bit, but there is still a good bit of traffic on the section. enough to warrant 4 lanes.
Peotone Wilmington Road, if the Illiana Expressway is not built.