Nikon can't really improve on the f6 too much or the mechanical f/2/3 too much.
If they are imaginative, they could do a couple of things - launch one or two F series anniversary editions - Nikon's own & maybe the F's anniversary -whatever those numbers be.
I don't think the current f6 can be improved much with existing design - an interesting and useful idea would be if Nikon can make an F6+ which can hold say 2 rolls of film instead of 1 - keep everything same but when first one gets over, roll in the next roll - some bit of ingenuity could work there.
Second The f/2/3+ version could be the one that's marketed to the hipster/lomo crowd with the 'retro' badge but engineered to the current technology materials. With possibility of running with or without batteries, and capable of using the older lenses as well. (The Df's film version, somewhat)
The pricing and support would be key. It's quite stupid that Nikon doesn't even look at film cameras here!
Maybe they could just announce reopening of Service lines for all the F range cameras - would that be a profitable thing to do? ;-)