I never really played an illusionist and you will probably lose quite some of the benefits of gestalt characters if you want to fully focus on illusion but possible class combinations I could think of would be Puppetmaster magus+ School savant Arcanist or wizard, with high charisma and high intelligence.
This gives you 2 class features that boost your spell Dcs, some swift action spells, access to the bard, wizard and magus spell lists, a huge number of spells per day and 2 ways to maintain your illusion spells without concentrating.
If you are worried about affecting undead with your mind-effecting spells, you could also take levels of dirge bard+ Sorcerer.
There really aren’t that many classes that boost illusions so you will probably end up with a lot of overlapping class features.