I found some interesting ones from Mississippi:
Biloxi to Jackson, TN All of MS 15 (it would've followed other routes to connect with its southern segment), TN 125, and TN 18 to end at US 45. There was also a proposal for a slightly different route that would instead go to Pascagoula, MS.
Hollandale to Homerville, GA. It would've used MS 12, MS 19, AL 10, and GA 37. When this was rejected, it was reapplied for only MS 19, AL 10, GA 37 from Philadelphia, MS. I've actually proposed making this corridor as a US route, I had no idea it was done in real life.
Avondale, LA to Crystal Springs MS 27 from Crystal Springs, LA 25, the Lake Pontchartrain Causeway(!), 90 BUS. (!!) to end at US 90.