Cause at the moment, as I'm sure you're aware, our sole DC-based Illusion spell is Phantasmal killer. Unless you count Hypnotic Pattern, which a. has an underwhelming effect, and b. causes a horrible screen-overlay effect which means no one uses it because they get squelched for doing so. All other illusion spells are just buffs, and hell there's not a whole lot of those: 2 flavors of concealment and 3 flavors of invisibility. And okay, we got race-restricted Color Spray (and illusion-based Symbol of Stunning/Flame, but as copies as existing spells is it really right to count them)?
Just, not really getting a "Earthen Illusionist" vibe from Deep Gnome. At all. I mean... two spells? Gnomes get +3 illusion DC. Deep Gnome IPL stance gives +3 Illusion DC, +6 to PK DC is pretty good but there's still no reason to build for illusion beyond just taking that IPL stance and AP. Someone interested in DC-casting is still gonna go Necro cause you get a similar single-target at-range insta-kill, but you also get Wail and Circle of Death.
Two (2) spells. TWO. Compare to the nicely fleshed out schools of Necro, or Enchant, or Evoc, which have multiple spells that do lots of things. We have enchant-specced, necro-specced, evo-specced, and conj-specced, even.
But then, for an "illusionist," there's... two spells. I could bind Phant. Killer to Mouse1, and Color Spray to Mouse2, and the entirety of the "illusionist" archetype could fit on my mouse (and if you really want to count Hypnotic Pattern, any mouse worth using has a center button).
Just to reiterate: TWO SPELLS.
Is there ANY sort of plan to introduce more illusion spells? Or is the purpose of the bonuses to illusion DC solely to make insta-kill DC-casters marginally more effective?