it doesnt make sense in oblivion/morrowind that you can simply summon a skeleton from what appears to be a plane of oblivion! bones from earth go to oblivion to be summoned back to earth? i dont think so!!
i think all physical undead shouldent be summoned randomly.. but instead you are given a SPELL that you can cast on corpses to put a friendly spirit in them!
this means that you can kill characters to gain allies. how interesting some possibility could be? you could assassinate a character, raise him, get him to fight the town guards and his death will be blamed upon the guards rather than you!! perhaps you could run from a giant ogre into a tomb full of skeletons, spray your raise dead spell everywhere and watch as your newfound army overwhelms the ogre!!
different corpses should take different amounts of magika from the player. a regular human,skeleton or mud crab will not need much.. a dragon or troll will. secondly casting the spell for longer on a corpse should make the corpse stronger. (and of course, your skill with conjuration)
this would also help greatly in making a difference between undead summons and daedra summons! daedra summons would remain the same. (this would not effect ghosts)
Soul release
what if you could release the spirits of your soul trapped npcs?
ok.. what if soul gems were equipable? empty soul gems in your hand would automaticaly absorb any creature you kill's soul!! when you have a filled soul gem in your hand you may release the soul from it so that it may fight by your side as a ghost,
ghost summon
this would be a spell that calls a random type of ghost to you although one suitable for your location (not so many ice wraiths in the city).. if you are lucky you might get several ghosts are a dangerous type.. if you are unlucky you might get a weak ghost, a ghost you cant control or a few props might fly around.