100% love this idea
Imagine you go into an Inn, talk to the “Stable Master” who is much more sinister looking to Death Knight eyes. You open your pet panel, and select a Lich from the Western Plaguelands you dominated whilst ganking some lowbies to improve your self esteem.
BAM Timmy crawls back into the dirt where he belongs and your Lich BFF portals in. You check his skills. Frost Bolt, Teleport, Frost Shield and Frost Nova. Oh yeah much excitement!
You accidentally press Dark Transformation trying to wipe the drool from your keyboard, you notice the new empowered abilities. Frost Blast, Silence, Ice Block and Deep Freeze.
The undead “family” determines whether it is a melee or ranged unit. Dark transformation won’t change your pets appearance and instead just makes it larger and all shadowy.
You sit back in your chair and think of all the other cool undead populating the world of Azeroth and beyond. You mount your trusty steed and go searching for the newer, rarer undead to add to your menagerie.