'Marketing' suggests we want people to buy it. Interest in film seems to go together with interest in 'retro' manual cameras rather than later auto models more often than not. Something in the Nikon FM series would sell. Ideally, an FM/FM2n hybrid with the wider shutter speed range, faster flash sync and removable focusing screen of the later model, but the pre-AI compatibility of the earlier version. Restore metal parts that became plastic in the FM2. Get rid of the requirement to pull out the film advance lever to the standoff position to make the camera work; instead, bring back the collar around the shutter release from the early FM variant and have it both unlock the shutter and turn the camera on, but key the meter to a half-press of the shutter release as in the FM2 to save batteries. No model branding on the front - keep the understated FM look.
If there were a market for an 'F7', combine the best features of the F5, F100 and F6. Removable prism and choice of finders, 4 x AA with an optional CR123A holder like the F100, optional grip with additional batteries, secondary shutter release and maybe an integrated Arca plate, flippable AI tab for pre-AI compatibility as standard, add AF-P and 'E' aperture compatibility so that virtually any F mount lens can be used. AF module from the D6. Obviously make compatible with iTTL/CLS like the F6. Not sure it needs the F6 rear LCD, but maybe have databack capability to allow imprinting between frames. Add a USB-C socket from which shooting data can be downloaded with a standard cable.
Slightly further away from the more rarefied realms of fantasy, our lottery winner might ask Cosina if they are still able to make SLRs (they were selling the Bessaflex until 2007, and the FM10 until quite recently). How about a Photrioflex?