Want a crazy suggestion?
I-80 is way too far south for its number in NV/CA, so put I-80 on current I-84 and renumber the current CA/NV/UT I-80 as I-52, I-54, I-56, I-58, I-60 or I-62. Then the freeway south of I-82 has a smaller number and all is good. Of course, you had to spend millions renumbering I-80 as well as every I-x80 in CA....
Seriously, I-84 should have been I-82, but since it wasn't then I'd go with the odd number for I-82. I'm not sure an I-x90 or I-x84 would be appropriate for what functions like a mainline interstate, not a spur.