Make it able to pierce disguises of polymorphed characters and distinguish between a PC and said PC's projected image, as per PnP.
That makes it a really super powerful though, so to balance out this amazing buff, take away the ability of true seeing to automatically detect stealthing characters and instead just give them a big bonus, like +30 or +50 or something, to spot checks for detecting stealthing creatures (but not +30 to spot checks for disguise).
Buff true seeing
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