At the moment, a couple schools of magic are kind of forgotten about even though they could have a great place in the world of DDO. In DDO right now the only schools of magic that you see people specializing in are Enchantment, Evocation, Necromancy, and maybe Conjuration. Abjuration, Divination, Illusion and Transmutations spells are almost forgotten about except the couple of spells that are used like Stoneskin, True Seeing, Invisibility, and Flesh to Stone. Illusions are the most forgotten about and easily can be fit into the game.
As the game is right now, there are 7 illusion spells. However, Illusion spells fall into that weird category that everyone believes are simply "Role Playing" spells when they aren't. In my mind the illusion spells should be hate grabbers that draw the fire away from the caster and maybe the rest of the party when they don't have a tank like character. For example take my example of silent image.
Silent Image (1st level spell)
List of Creatures: Kolbold Warrior (runs towards them), Kolbold Shaman (pulls them away from you), Kolbold Thrower (pulls them towards you)
The illusion creature will attack (like a summoned monster) drawing hate to the best of its abilities. It doesn't produce any sound so until the creature see it they won't go after it. However when they first see it they are entitled to a will save to disbelieve the creature and gain another will save every time they strike it to realize it is an illusion. If they make a successful attack against the creature (making the will save) the illusion poofs and is destroyed.
Minor Image would allow some sounds so they could potentally be used to draw creatures towards you as well as adding a different list of creatures that you would normally find at level.
Mirror Image would have 1d4+1 illusions of yourself run from you and start casting spells (Scorching Rays Animation) to draw hate away from you.
The other "Image Spells would follow this principle seeming to become more real every spell.
Now not all illusion spells are focused on creating images but making your image, disappear. Spells like invisibility sphere and Greater Invisibility would allow people to fade from the world as the illusions grab and pull aggro. Invisibility is already in the game as well as its mass version so imagine these except shorter and don't go away after attacking like the Mabar cloaks.
One of the greatest things about illusion though is its versatility though, because when you get to higher levels you can use a single spell slot to cast several different types of spells. Shadow Evocation and Shadow Conjuration allow you to create several types of evocation or conjuration spells without stuffing your spell slot full of them the only draw-back is, they are only partially real meaning that if they make their will save, they only take 20% of the damage. Look at Shadow Evocation for example,
List: Shadow Force Missile, Shadow Fireball, Shadow Lightning Bolt, Shadow Frost Lance
You replicate the effects of the spell by the same name however the target is allowed a will save to disbelieve before the effect takes place. If they make their will save, they only suffer 20% of the damage that the spell would of normally dealt. Stuff like spell resistance and additional saves like reflex saves are allowed to the target of the spell whether they make the will save or not.
Shadow Conjuration would be the same thing with a different list of spells 3rd level or lower.
Greater Shadow Conjuration would be 60% real with a list of conjuration spells 6th level or lower while Greater Shadow Evocation would be 60% real with a list of Evocation spells 7th level or lower. Shades would be the Greatest version of this spell allowing any conjuration level 8 or lower and being 80% real. Where these spell might not seem that great for wizards, sorcerers would gain some more spell but at the cost of extra saves and bards would be able to cast some spells not usually on their list.
Nightmare and Weird would be the last spells that I would like to see added into the game. Nightmare, would be changed from pnp that only did 1d10 damage and caused fatigue over a night's sleep to any creature asleep would be dealt 1d10 untyped damage every 2 seconds they were asleep making it a powerful tool to anyone who can use the sleep and deep slumber spells that are right now undesirable spells.
While Weird was a mass Phantasmal Killer.
Just my thoughts.