Wow, well this is what I woke up to
Anyway, in short, you can actually lift the race restrictions for Dark Rangers to become a playable option. For some races.
To a degree, this has already been done. Like others have stated, there are not only former Elven Rangers
whom have been raised as Dark Rangers.
You also have the newer ones who were raised at Darkshore.
And, sort of, the Forsaken Rangers trained by Nathanos. Note though that, while they are described as having some ties to Dark Rangers, they aren't actually true Dark Rangers themselves.
Is that last part a big deal? Depends on who you ask I guess.
However, we still have the issue of the Undeath-requirement in order to become a Dark Ranger.
Technically, sure, their type of magic is not exclusive to being undead. You could just overlook that requirement.
My question is: Should you?
Because, if you ask me, if you lift that restriction then you would no longer fulfill the Dark Ranger-fantasy.
You wouldn't actually have a Dark Ranger anymore.
Because part of their fantasy, is being former resurrected rangers.
There is one class that can be played by a lot of different races. There is one class that when chosen, automatically makes you undead.
The Death Knight.
They already have the connection to necromancy and the manipulation of the life and death-essence.
Teach them how to fire a bow properly and you're golden. Sort of.
The biggest obstacle is the current armor-type associated with the DK class.