Summons for Illusion could use some love. Greater Shadow Conj is really it, and while it's quite useful at lower levels, it falls of quite heavily. GSC is a 6th level spell, so should not compete with 9th level or epic level summoning spells, but that should not limit our creativity to support Illusion summon potential.
This being said I propose several alternatives to provide more interesting AND useful summons for illusionists.
1) Add a minor buff to make the current 6th level GSC slightly more useful. Specifically with low damage it fails to pierce common mob DR. It could use a slight AB and HP bump perhaps as well. Concealment is fairly high as it is so it tanks well considering, but that's mostly against basic attacks.
2) Add a higher level illusion summon spell (9th and/or Epic) to provide an alternative for more powerful summon that scales off of Illusion foci and is "investment appropriate" compared to alternatives in Coj & Necro (e.g. Mummy Dust, Planar Conduit, Summon Creature IX).
3) Provide some flavor/mechanical benefit for not just ESF Illu, but perhaps even Illu spec or an Epic Feat in terms of higher tier summon or various streams (a la Dread Mummy). Bonus points for Shadovar specific cookies here as well, that would be interesting as long as it was toggleable (like a special stream) as to not be metagamed.