The Death Coil warlocks had is very much a variation of the Death Coil from WC2 and WC3. The only thing the warlock's Death Coil has to it, is it fears. Well, that is warlock for you, their CC is generally fear so duh. It was renamed to Mortal Coil to end the confusion between the two spells which was introduced in WotLK by introducing the Death Knight's Death Coil.
http://classic.battle.net/war2/units/deathknight.shtml
Here's the original Death Coil from WC2.
Also here's art from the DK from WC2. Every DK drove an undead horse. I think its inspired by Nazgul (or Buckriders, a legend near the city of Maastricht and Liege), but which fantasy isn't.
Now, of course the DK mount is kinda similar, but DK aren't always on their horse.
As you can see by these examples, Blizzard will deviate from "past lore" and "past Warcraft games" if they see fit.
---------- Post added 2013-01-29 at 01:11 AM ----------
Doesn't sound bad at all, if there's dragons in the expansion, and if it can use guns and (x)bows. Don't see why not!