It's not just DDO that has fewer illusion spells then other schools, but DDO only has 8 illusion spells? (only 2 of which allow saves) That seems very limited. So here's some ideas I drew up while looking at some D&D spells that weren't included: (Some modified to be a little more useful for DDO)

Silence (level 2 bard/cleric/favored soul): Long range, small AoE, prevents spell-casting in the zone. Will negates. Spell resistance applied. Source: http://www.dandwiki.com/wiki/SRD:Silence

Nightmare (level 5 bard/sorc/wizard): Puts the target to sleep as if effected by the sleep spell (20 seconds + 2 per caster level), while they are asleep they take 1d10+1 per caster level damage immediately and every 2 seconds they remain asleep, this does not wake them up although they do awake from other attacks. When they are awoken they are fatigued for 2 minutes. Source: http://www.dandwiki.com/wiki/SRD:Nightmare_%28Spell%29

Simulacrum (level 7 sorc/wizard): Target a creature and summon a shadow version of it. It retains all special abilities of the targeted creature (so if you summon a copy of an earth elemental, it will use earth grab) Cannot use on red named bosses. It would also be good to get a simple action bar for this creature (active/passive, use obect etc, like a hireling/pet without special abilities) to make this worth the 7th level slot. Source: http://www.dandwiki.com/wiki/SRD:Simulacrum

I would also suggest weird to give illusionists a 9th level spell worth getting... but weird looks EXACTLY like phantasmal killer except for: spell level (weird is level 9 sorc/wizard), damage if it doesn't kill, weird also deals str damage if it doesn't kill.