|Golden Flower Granny|
In Demi-Gods and Semi-Devils, disguise is taken to extremes. In one instance, the hero thinks he is fighting a duel to the death with the man who killed his parents, but in reality it is his wife disguised as his opponent. The reasons for this are somewhat complicated but the end result is a tragedy of Shakespearean proportions. In Heavenly Sword and Dragon Saber, we meet a character named Golden Flower Granny, who is actually a beautiful sacred maiden of Persian Ming Sect disguised as an old hag because she broke one of her vows and had to flee. These kinds of big reveals happen a fair amount and occasionally they can be useful in a campaign (as long as everyone and his brother isn't constantly pulling of his face to shock people with their true identity).
Disguise is a handy tool for the GM during play as well. Just as the player characters can masquerade as others to gain access to secrets and secure areas, their enemies can and will do the same. If things get convoluted enough, maybe even their allies will end up tricking them with disguises.
This is one of the reasons I made the Secret Identity Tables the other day (HERE). But secret identities need not be the product of random rolls, they can be deliberate and planned in advance. I wouldn't suddenly decide three months into the campaign that their manservant is actually Lady Plum Blossom in disguise. But I might decide that when they first take the manservant into the group, if it is applicable.
Here are three ways disguise might factor into your campaign.
Someone Tries to Infiltrate the Party
This is most likely an enemy, but it could be an ally. It may also be a minion of the person in question. In this sort of scenario, the person disguises themselves as someone the party would be favorably disposed toward (not necessarily a specific personally but a type) and makes arrangements to meet the group by 'chance'. Maybe they show up to pay an expensive Inn bill when the player characters are low on funds or they stage a situation where the party rescues them. The purpose of the infiltration could be anything from information gathering to assassination.
Someone Tries to Mislead the Party
This is a classic tool for villains who love to turn their enemy's strengths against them. Perhaps the party is trying to find the murderer of their master and they are told that the head of Sun Mai Sect, Abbot Yuancheng, knows the killer's identity. The Abbot is beyond reproach and highly respected so the players think they can trust him. Little do they know that the real killer has disguised herself as the Abbot and arranged to be at the Temple when the real Abbot is away on a secret mission. When the party arrives and asks who killed their Sifu, she pins the blame on one of her other enemies.
Someone Turns the World against the Party
The players themselves don't have to be the direct victims of the deception, they can suffer indirectly by having their reputation harmed. A person with a grudge against the party could disguise himself as one of their members and go on a terrible killing spree. In some cases he may not even need to kill to do harm, he could simply rack up huge debts or violate several of their sect's rules.
Adjudicating disguise can be tricky. There are two basic points where the disguise can fail in the Wandering Heroes of Ogre Gate System, other systems will deal with this differently of course. This is how I handle it in my games. Suspicious characters can make a Detect roll to see if the person is disguised. The TN is set by the target's initial Talent (Disguise) Roll and you should only ask for the Detect roll if the player says he or she wants to take a closer look at the person (or if the initial Disguise Roll was particularly bad). Beyond that the disguised character still has to perform as the person they are pretending to be and this Requires a Deception Roll against the player's Wits. If they fail the roll, you would inform the player that something about the person seems off. They wouldn't necessarily know that it's an imposter, but they would be able to tell that something just doesn't seem right.