Using the D850 for wildlife, my ideal would be to hot-switch between 1 of 2 metering modes together with one of 3 different AF mode(s). The camera uses Matrix metering as default - so effectively this totals 3 options. The current options allow BBAF > a9 or a25 (usually), Pv > single-point AF; and the AF Joy-stick/Subselector > AutoAF (or GroupAF). Currently, depending on the selected Custom settings bank, one of 2 metering modes is assigned to Fn1: usually I use spot-mode, but AEL is also very useful in backlit situations. Ideally, these respective metering modes would work better as instant options.
Switching Pv on Single-point AF, I also press the metering mode with the ring finger; as one's thumb drives BBAF or the Joystick. Depressing the joystick is fiddly but it's the only control available).
As we know Nikon designed the D850 and D500 without the Fn pair (Fn2 and Fn1). This absence limits customizing these cameras, and obviously cannot be upgraded with Firmware. A firmware fix on the D5 and D6 would allow assigning a hot-switching 2nd metering mode to camera-front (ie Fn1 and Fn2 working the alternative modes). Ideally, it would be useful if Nikon added an extra control on the rear-plate of all their Pro cameras. Adding an AE D780-style control would work, and even better if it worked as a switching toggle labelled "Alt- AF" to toggle between 2 AF modes under one's thumb(?) - thus Left vs Right press. There is sufficient space, and this would free up the front controls for choices of metering modes etc. A less ideal solution might be to improve the AF Joy-stick to work better in its 2 distinct modes (lateral / depressing), but still only able to assign the choice of one AF mode.
