Fair enough. Changelings certainly bring fun to the table.
And I decided, if you (or anyone else) pick Warlock, what to do: just make them a little more versatile, easy to use in many situations: all Invocations are Supernatural abilities (no ASF, no attacks of opportunity, no spell resistance, save DC is always 10 + half level + Cha), pick Cha+1 Cantrips or Orisons, you get them as bonus (Su) abilities at will for party tricks or whatever, and they also get the Witch's Curse as a (Su) at will:
Range: melee touch attack
Saving Throw: none
Duration: until the next corner of the day (dawn, noon, dusk, midnight), at level 10 this becomes "until the next full moon, in worlds without moons (like Sigil) that's just 30 days), at level 20 it becomes "permanent until Wished/Miracled".
Effect: select one:
*the target suffers 20% action failure on all actions
*takes a -4 penalty on all d20 rolls
*becomes fascinating to animals and vermin, who are attracted to the character within a 1 mile radius and are all determined to share the character's occupied space.
At level 3 onwards, gain these options:
*the target suffers 50% action failure on all actions
*takes a -8 penalty to one ability score
*all of their equipment instantly falls to the ground, even things that really shouldn't fall off. Even bags spill open.
(Yes, at level 3 you can go around making people's pants fall off. Because Warlocks are like that.)
Level 5+ option:
*the target suddenly ages, taking a -6 penalty to Strength, Dexterity and Constitution and has poor vision, gaining a -6 penalty to Spot and Search and treating everyone as though they had Concealment.
Level 7+ options:
*the target is rendered unable to speak
*enters a Rage but has a 50% chance of not being able to tell enemy from ally.
*they become terribly allergic to either Plants, Animals or Vermin (caster chooses), taking a -10 penalty on all attack rolls, Fort saves and skill checks when within 10' of one.
Level 9+ options:
*the target forgets a single class feature of the caster's choice
*is treated as having zero ranks in any of their skills
*a Centipede Swarm appears within their stomach and attacks, requiring disembowelment (50% of their HP or more) to remove. Every morning, another swarm will appear there. Oh joy. Replace with Hellwasp swarm at level 15+.
Level 11+ options:
*the target is rendered Blind and Deaf and also enters a Rage, charging in random directions and attacking whatever they bump into
*their clothing all animates (as per Animate Object) and tries to kill them.
Level 13+ options:
*the target becomes suicidal, attempting to find ways to end their own life
*is rendered unable to sleep, suffering all of the effects of sleep deprivation.
*their hands turn into Small Monstrous Spiders (with HD equal to the Caster Level) which immediately try to bite them until hacked off (dealing the damage of the injury to the target) and then slain. They then turn back into hands, but are detached. This will happen every morning for the duration, as well.
Level 15+ options:
*the target becomes Stunned or Nauseated
*despite all logic is targeted by a single Comet once per day for the entire duration. What are the odds?
*they contract a disease, and contract another one every morning they wake up.
Level 17+ options:
*the target suffers an Eternity of Torture effect
*their weapons turn into snakes (Half Fiend or Half Celestial, with +1 Outsider hit die per Caster Level) and immediately attack them, treating their natural weapons as though they had all the special abilities and enhancement bonuses of the original weapon. If it is a weapon of 3 Wishes, you can expect the snake to waste them, going "I wish I was out in the desert somewhere. I wish I had a pond of cool water. I wish I had a mouse. Yeeeeeeah, I'm so happy."
And for the record, yes, Sigil Prep will still be 50% curricular activities (school-mandated dungeon crawls), 50% extra-curricular - players going about and finding/making trouble, messing in the affairs of each other and NPCs, and so on. Expect the penalty for basically any rule breach, or for failing something, to be a spanking, some form of sexual indecency, or both. Possibly at the same time.