About 2/3 or 3/4 of Tennessee's 2DI's that aren't already six-laned or more due to heavy traffic (especially trucks), in order of priority:
I-24 between Murfreesboro and Chattanooga
Entirety of I-75
I-65 Nashville to Kentucky border
I-40 Memphis to Nashville
I-40 Nashville to I-81
I-81
I-65 south of I-840, I-24 west of Nashville, I-40 east of I-81, and I-26 don't need to be six-laned, they are fairly lightly travelled. And I think I-55 and I-69 (what exists of I-69) are 6+ lanes except for the Mississippi River bridge, which would require an entire new bridge replacement (and a new interchange replacement), although I could be wrong. I don't think expressway sections of US 51 between Dyersburg and Fulton are signed I-69 yet anyway.