You
will not be able to use the cache to detect the appearance/existence of mobs near you, period. It does not matter whether they're rare or not. There will be no automated background scanning for specific mobs once 6.0 hits.
You
will still be able to detect when a vignette appears, but those only exist for certain rare mobs from recent expansions. They're also not always reliable.
Other passive "scanning" methods that will still exist include watching the combat log, watching nameplates, and watching what you mouse over, but those are basically useless.
Unless Blizzard adds vignettes for rares from older expansions, the main method you will need to use to hunt for specific mobs in WOD will be the use of a /target macro, just like you used before NPCScan came along.
I don't know if there's any plan for NPCScan, especially since it's author abandoned it some time ago and Torhal really only signed on as a maintainer to keep it going as-is, but I use
SilverDragon, which already uses all the methods I mentioned above, and provides a macro/keybinding to run a /target macro tailored for your current location.