Waagh beat me to my best points. To tack on a few extra thoughts:
* 6+
FNP rolls just not being worth it is probably why the remaining
FNPs in the game tend to be 5+.
* I'd be worried that giving everyone a pseudo Reanimation Protocols rule would water down the uniqueness of necrons. Ideally, I like each faction feeling unique. Plus, a lot of medic characters already have some ability to revive dead models (usually in the form of a stratagem). See: apothecaries and haemonculi. At least, I *think* there's still a strat for reviving dead guys with haemonculi.
* Is
FNP worth buffing? I feel like most of the 5+++ versions are already pretty decent, and the replacements Waagh pointed out for orks/sisters seem like decent prototypes for rules that accomplish the same thing but with less rolling.
(This also goes back to the start of 8th, where in promotional materials they were talking about how cool it was that you could take a Venerable Iron Hands Dreadnought who would get two 6+ FNP saves, and then within about three months had changed it to, 'We never intended for you to get multiple FNP saves'...)
Ah yes. I remember that. Given the game's current lethality, would it perhaps be reasonable to let
FNP type rules stack? So for instance, being Iron Hands and being within 6" of an Apothecary both give you FNP6+. So perhaps being Iron Hands in range of an Apothecary could stack to give you
FNP 5+? No more rolling than a single source of
FNP, but you'd still have a reason to include something like an apothecary in a list that already has
FNP from some other source. Would have to put some limits on it to avoid death stars though.