Sorry for any repeats, but I have a few suggestions:
KLXV - Leadville, CO
KSBS - Steamboat Springs, CO
KGNB - Granby, CO
KEEO - Meeker, CO
KRIL - Rifle, CO
00C - Durango, CO
KGFZ - Greenfield, IA (Home of the Iowa Aviation Museum)
KAFK - Nebraska City, NE
50K - Pawnee City, NE
KFET - Fremont, NE
All small or small-ish airports that I think would be awesome additions!