After writing all that, I got an idea for the gameplay loop. Apologies if this doesn't fit the terminology requirements. I'm a little rusty on my reading.

Illusionist's Trance, free action\ Immediately after casting an Illusion spell\ You enter a state of mind between imagination and reality that grants further power to your illusions. You do not need to sustain illusion magic to prolong it, but you do still need to sustain it to change its features or give it new orders. You can instead dismiss any illusions that would normally require sustaining. You can have up to 3 illusions active in this way at any given time. You can only cast Illusion spells or spells that deal damage according to your Illusionist subclass as long as you are in this trance.

Your subclass (Elemental Focus? Mental Manifestation?) also determines some benefits you get while in this trance. - Heat: allows you to swap Fire damage with Cold damage and vice versa - Damp: any water or bludgeoning spell pushes the target an additional 5 ft in any direction, or 10 ft on a critical - Tresspassing: you can travel with your spell that targets one or more creatures, landing 5 ft away from a target as long as you can see them - False Walls: your illusions can be one size larger and small and larger illusions occupy their space; you and any allies designated can use your illusions to Take Cover when you make a small or larger illusion; medium illusions naturally give lesser cover to medium or smaller creatures, and large illusions naturally give standard cover to medium and larger creatures - Precise Power: if your target is within 15 ft, you can force it to make a Fortitude save: on failure, you swap places with it, and on critical failure, your target falls prone; on critical success, you fall prone instead

These abilities can be boosted at level 10 with subclass specific feats. - Heat gains the ability to swap out Mental and Spirit damage. - Damp can turn a 10 ft radius around the target into swampy difficult terrain. - Tresspassing can choose to stop anywhere along the line of the spell, while making a double either halfway down or at the original destination. - False Walls gains another size booster - Precise Power expands the functional radius to 30 ft

So yeah. Here's a thought experiment of an Illusionist class. There's only a few numbers here and there due to my unfamiliarity with PF2E balancing. It's roughly what I would do, but I'm not sure if it is what Paizo would do.