While I like the Prestige classes being filled out with more flavor, makeing ALL Chameleons assassins could spoil the PrC for others.
There are several different ways the Chameleon can be introduced aside from the way listed;
Schisimed Mind: The Chameleon is a person with multiple personality disorder. The different focus represent different personalities. Sorak from the "Tribe of One" Dark Sun novels would be a good example of this. A Telepath might be able to do this by way of Schisim & a reverse Mind Seed.
Shaman King: Just like the Anime, this Chameleon is channeling spirits to change his focus. Great for Shukenja and Shamans.
Haunted Mind: One of my Ravenloft PCs had a bad habid of encountering ghosts and being possesed by them. What if each of them left a little behind and he gained limited access to their class abilities from the possession.
The Pretender: Like the TV show, your PC is simple a very good learner. Maybe he spent his childhood growing up in a library not even realizing what he was learning.
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It suggests that Psionics could be used as well with the Chameleon, but does not say how. Here is how I would add a Psychic Focus.
For a Psychic Focus I would add an additional requirement, the Hidden Talent feat from the XPH, pg67. Normally, this feat can only be chosen at 1st level, but a Chameleon may take it with any Chameleon level.
Once he has this feat, he unlocks these special abilities:
Psychic Focus: Power points and Powers known would be as a Psion level 1 to 10. You gain bonus Power points from high Intelligence. Maximum level known would be 1st at first level 2nd at second level and third level. and 3rd at forth & fifth, 4th at sixth & seventh, 5th at eight & ninth and 6th at tenth.
Powers may be chosen from the list of any Psionic manifesting class. Powers known are locked in for each Chameleon level. Whenever you gain a new Chameleon level, you may swap out a number of powers equal to your Intelligence modifier. The new power must be the same level as the power being replaced.
When you assume Psychic Focus, you automatically achieve Psionic Focus durring the meditation.
Bonus Feat: You may now choose any Psionic Feat you qualify for. When you have Psychic Focus in place and are at least 3rd level, you can use this feat to take Expanded Knowledge and choose an additional power.
Mimic Class Feature:
You may now add two additional class features;
Wild Surge (Su): As per the Wilder, when you surge, you gain +1 to your Manifester level and may automatically augment the power you are manifesting by +1 power point. As a Wilder, you are also subject to the risk Psychic Enervation whenever you use this ability.
Your Wild Surge increases to +2 at 6th level and +3 at 9th level.
Mind Blade (Su): As per a Soul Knife, you can create a Mind Blade. It is considered a +1 Short Sword of Psychic force.
Your Mind Blade increase to +2 at 6th level and +3 at 9th level.
Psychic Reformation (Psi): At 10th level, you gain the ability to use this power once per day at no power point cost. You must have Psychic Focus active and you must spend the neccessary expericence points to make the changes you wish.
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You might also create Chameleon feats that unlock additional Class Features to Focus & Mimic, especially for any of the additional core classes we have seen; Healer, Warlock, Courtier etc.