icky undead hybrids... they wiped out several empires
Mindstab Thrull makes you forget spells and psionic powers.
Necrite Thrull kills ya ("Have you ever seen a wall drop dead of fright?")
Armor Thrulls remains are natural bone armor
Basal Thrulls serve as a power battery for necromancers
Thrull Champions make other thrulls tougher
The Derelor massive thrulls that stand up to giants
Orggs
Giant monsters that think they are little monsters.
Heh... yes, there are many creatures from MTG that would be interesting in 3e. How about Thallids?
FYI, on the Realms of Evil site in the Womb of Creation forum, they have recently been creating 3e conversions for many MTG creatures, such as slivers, a sengir vampire template, nightstalkers, and so on: