My Mamiya Universal with f2 lenses, the Zeiss 110 Planar and Schneider 80 Xenotar, (the Rollei versions with shutters, not the lame shutterless Hassy 110), perfectly coupled to the rangefinder of course, and with appropriate brightframes replacing the factory ones.
I know they probably won't cover 6x7, but open the mask up enough so I can choose 66x44 or 56x56 or something in between after I shoot, on a 6x7 neg.
Give it a projected reverse-rangefinder option like some of the Graphics had, and an interlock that would extinguish the light right as you fired. Make it a green laser, so it would have longer range than the incandescent solution in the Graphics, and yet wouldn't cause the oh-shit-a-sniper reaction you can get with red laser spots. An intensity dial would be good too.
Make it auto-advance slowly and quietly, without giving up the great film flatness of the non-reverse-curl Mamiya backs.
This is all probably possible, although getting the helical pitch/rangefinder sync right might be tough.
For pure fantasy, I'll dream a 4x5 "overgrown Universal" with a 150/2.8 Xenotar. (It'd probably have to be a design from scratch, everything's just barely in the wrong place to let a real Universal be adapted to full-frame 4x5) It'll have a 5 inch roll back that somehow still lets me get my eye to the eyepiece. Best of all (since this is a fantasy) there'll be Delta 3200 loaded in it.
Another fantasy would be an 8x10 version of the Cambo Wide. I wouldn't care so much about movements, but I'd need real scale focusing, not the preset hyperfocal of the Hobo. Also, a 120-degree lens that's just a little too short to cover, like my 35mm APO Grandagon on 4x5, so I can get a 120 degree angle of view with any aspect ratio I want.
, not cheaper. 
