One way to do illusionists is to use the alternate class hosted on autarch.co (I swapped out constructs for phantasmal creatures, however). Below is a different alternative class, which requires the Player’s Companion.
Illusionists have the same class build as mages: 4 arcane caster, 0 for everything else.
Illusionists can perform magical research as mages at 5th and 9th level, and build a sanctum as 9th level. At 11th level, the illusionist can perform magical research for ritual illusion spells, and create phantasmal creatures, but cannot craft constructs, cross-breed creatures, or work with the undead.
An illusionist’s sanctum has a special option: part or all of it can be quasi-real. Any real portions must be constructed first. The quasi-real portions cost the same amount, but take only one day per 5,000 gold value to construct, and do not require laborers or crafters (but do require engineers, and are still limited to realistic architecture).
Quasi-real structures cannot be “disbelieved,” but dispel magic can open a hole for 1d6 rounds before the structure re-asserts itself. Quasi-real structures can repair themselves at a rate of 5,000 gold per day while the illusionist is present, and have no monthly maintenance cost … but lose 0.5% of their value for each month the illusionist is not within one mile.
Illusionist spell level multipliers differ from those of the mage:
Enchantment: x0.66 (hold x1, affect undead x0.85, affect animals x1.33)
Summoning: x1 (x0.66 illusory summons)
Edited: changed illusion from x0.5 to x0.66; otherwise an exceptional roll can produce phantasmal killer as a level-1 spell, which is a bit much.