That Bugatti would be nice..they do pop up on eBay occasionally (the '78 issue is usually pretty reasonable). A bunch of older Monograms I wouldn't mind seeing include the ex-Aurora Maserati 3500GT, Bugatti, the MG TC, '70s Merc 450SL, Land Rover (though I have one of those), Porsche 924, Mazda RX-7, Chevy LUV pickup, and some of the '30s classics (though those don't seem to sell well). All kits that haven't been out since I was a kid.
For Revell, I'd like to see the Porsche 356 and 914 and some of the '80s Revell of Germany Porsches (911 Targa particularly).
A more recent kit that would be nice to see reissued is the Essex Wire race Cobra.