1st: Specialized clothing that attempts to hide the wearer. These can come in several forms, each giving a better chance of concealment then the past level. Plastic masks, skin gloves, pigment dyes, voice changers, etc. All the way up to full body coverage suits with holographic disguises for face and voice. The disguise prompts the wearer when they put it on to change their name, description, and nakeds (if they don't change nakeds, then the naked messages are either ignored while the disguise is worn, or they stay... I lean towards the former.) Whenever someone looks at the person, it rolls to see if they detect them. If they do, it'll say: You notice that <name> is really <real name> in disguise. Or something like that. If they wearer is a good RPer, they might even drop clues or whatever to let players try and guess.
2nd: Normal clothing system can be tweaked, and an @disguise verb is added. That verb basically changes the user's name and description (allows for one disguise to be stored as well). Now, if they are wearing something (like say how Gally always wears her GRS suit) that they always wear, then the person knows its Gally and not Sue. Or whatever. Also does the roll against PCP when someone looks at the wearer, etc.
3rd: Now, this one is a bit different... If you really think about it, its not that hard to disguise yourself. A nice ski mask, full body clothing, and change your voice. Boom, you become Bob. If someone just pulls on clothing that has full coverage, and then uses an @disguise verb, it allows them to change name and desc. Then, if someone beats their disguise, they get a message saying that they discover <so and so> is actually <so and so>.
