Don't forget the Nilshai from UE - cast two spells per round. Add it to the chronotyrn, etc.
Every elemental healing ability is just ridiculous fun if used right: Iron Golem (Fire), Mechanatrix (Electricity) etc.
Getting fire and acid healing along with Regeneration would be fun.
Getting lots of different types of regen on the same creature could be interesting. Regen (fire/acid), Regen (silver/good/[good]), Regen (divine/blessed), Regen (electricity), Regen (good/[good]/holy water).* You'd effectively take non-lethal damage from everything except suffocation.
*Regeneration abilities from Troll (MM1), Devils (MM1), Beast of Malar (MoF), Fire Troll (Drag Comp), Abishai (MoF/FCII), respectively.
Getting awesome Natural Armor would be cool too. Medium Shadow Dragon = +19 natural armor.