A while back, there was a post on the City of Iron blog that casually mentioned how many of the new spells in Unearthed Arcana are illusions. I would say that this is because illusions hold the most promise for creating easy spell variants. Here's the basic recipe for a new illusion spell: start with a specific visual, auditory, or other sensory experience, add a specific side effect if the illusion is believed. An official example is the Shadow Door spell for illusionists: it creates an illusory door and makes the illusionist invisible when the door is "entered". The door has an illusory empty room "behind" it. An off-the-cuff idea for another such spell would be an illusory flash of light that causes a temporary blindness effect.
I actually started this post quite some time ago, and planned to work up a little random table for constructing new illusions. But looking over the existing illusionist spell list, I noticed how incoherent the list looks. Some simple spells have much more powerful variants with essentially the same usage and same spell level (compare Ventriloquism to Whispering Wind from Unearthed Arcana, for example.)
I think this post will have to serve mainly as a reminder to myself "fix illusionist spell list for my own illusionist subclass". I will point out, however, that you can create random illusionist spells by rolling a d6 and d4 on the Quickie Dice Tool sheet. The d4 gives the basic appearance: read vertical position as equipment type (right-hand side,) horizontal position as material type (bottom edge,) modified by the d4 result itself (use the four medieval elements.) The d6 gives the spell's function: vertical position gives equipment type or ability affected (right-hand side,) horizontal position gives basic action (top edge,) with the d6 result equal to the spell level. This will generate some pretty gonzo results; my challenge is to come up with a better way to generate huge varieties of random illusionist spells that still make sense.