I'd opt for a IIIg with an M mount, a Leotax T2L style rewind crank (it's much better solution than the canted ones on the Ms!), no self-timer, an M3 style shutter dial - maybe even with the linkage to an M meter - and the M3's frame counter and lever wind.
I'd keep everything else exactly as it was on the IIIg - bottom loading, no flappy back, separate RF/VF, 50mm framelines with 90mm nubs, and no meter. I'd even keep the top-mounted rewind/advance switch, and I wouldn't need a frameline preview lever, as there'd be no other framelines to view! Heaven.
That said, if we're in the realms of fantasy... an X-Pro style VF magnification switch could be nice, so you could swap from a 50/90 VF to a 28mm one with 40mm "nubs".