Summon Shadows in the early levels is very fantastic. There's a large lack of enemies with blind fight and offensive magic, so that 70% concealment carries you quite far. As you get into the mid levels, though, level 10 and up, the shadows very quickly fall into a ditch to never be used again, especially in favor of Undead.
Summon Shadows doesn't really get much scaling. Their weapon does no damage, but have a permanent +1 magic damage and +2 Physical, on top of a 1d6 Sneak Attack that never upgrades (3 shadows total for "3d6") and IKD. They gain +1 AB per CL (Assuming? Might scale off the caster class only), and that's about it. With only 14 ac, that 70% concealment gets lame pretty fast, especially once you run into enemies with magic.
I think an upgrade to what they excel at is all they need, a scaling on the sneak attack damage. Perhaps an additional 1d6 per 3 or even 5 levels, just something to consider.
They will still fall off I think, so an Epic summoning spell catered towards Illusionists could compensate once they're in the Epic Levels, perhaps something that mimics the spell Simulacrum, https://www.dndbeyond.com/spells/2618995-simulacrum
Creating a copy of a player, beast, devil, anything you desire and having it's stats, feats and skills wired to a specific number (Which increases with SF/GSF/ESF Illusion)
Another option is giving Shadow Dragon some special focus, since it's a rather lackluster stream. The summon could, in some way, be enhanced so long as you are an ESF Illusionist.
Or just another summon shadow that obtains even stronger sneak dice, higher AB and more DR, perhaps some feats that would help it. Just listing some ideas.