The problem I run into when going across country is either going to and from the same way or making a loop South to go, middle route coming back.
Since you've done the southern and middle routes your best option would be I-70 and I-90. I would go using I-90 and come back on I-70.
I-65 north to I-74 (stop at Indy Speedway)then northwest to I-39 North to I-90 west up through South Dakota (Mt Rushmore to the south) and then in Wyoming hit the Devils Tower and on to Yellowstone. Continue on west using I-90 to reach the Washington coast and travel down to California. Once you get into the LA area take I-210 east to I-15 and head north through Las Vegas to I-70 to head back home.
Sounds like a fun trip...
