US 6 from I-15 in Spanish Fork to I-70 at Green River. Yes, I know this was originally going to be I-70, but it probably doesn’t warrant an Interstate. It does need 4 lanes, and a few interchanges at high traffic intersections like US 89 would be very helpful.
In the Salt Lake City area, Redwood Rd (UT-68) has a couple two-lane sections that should be four, one north of the city and one in the south Salt Lake Valley. I think there’s currently a project going to widen the south section. Also, 1300 East should be 4 lanes due to the insane amounts of traffic on it at rush hour. It actually used to be 4 lanes, and then the city did a road diet, mostly due to pressures from bicycle groups.
US 189 should be four lanes all the way from Provo to Heber. There is still a two-lane portion near Deer Creek Reservoir.
Finally, although it would never happen due to topography and other environmental issues, both Cottonwood Canyons (UT-190 and 210) should be four lanes.