Probably why digital had to come along. Film cameras would kill film cameras! It appears almost nothing could be improved.
So in other words, are current cameras nearly perfect? Think about how huge changes occured between the F3,F4, and F5, and even among lower levels, you had strange oddities like the N4004 and N70 compared to everything else going on. But now camera layouts in the Nikon brand have largely remained the same, even when comparing the D40 to the D4.
I don't think the viewfinders on an "F7" would be interchangeable. I think that's a long gone option, otherwise there would be such a thing for the D(x) series pro cameras I would imagine.
I think the F6 is an F(x) though admittedly, I would agree that it might have always been intended for rich amatuers. I don't think too many pros were shooting film and even among those who were, I'm sure that Nikon would have see that digital was the way of the future. There are ton of F6 cameras on ebay right now with descriptions like "I only shot 15 rolls of film through it." In other words, the camera was great, the batteries died and I never got around to replacing them!