Base Class: Rogue
While Mastermind has very strong flavor, in practice it ends up not having much mechanical cohesion; this rework focuses on it being a social deception class at the front with combat features based on the overall theme of the class.
Master of Intrigue
When you choose this archetype at 3rd level, you gain proficiency with the disguise kit, the forgery kit, and one gaming set of your choice. You also learn two languages of your choice.
Additionally, you can unerringly mimic the speech patterns and accent of a creature that you hear speak for at least 1 minute, enabling you to pass yourself off as a native speaker of a particular land, provided that you know the language.
Master of Tactics
Starting at 3rd level, you can use the Help action as a bonus action. Additionally, when you use the Help action to aid an ally in attacking a creature, the target of that attack can be within 30 feet of you, rather than within 5 feet of you, if the target can see or hear you.
Soul of Deceit
Starting at 9th level, your thoughts can’t be read by telepathy or other means, unless you allow it. You can present false thoughts by succeeding on a Charisma (Deception) check contested by the mind reader’s Wisdom (Insight) check.
Additionally, no matter what you say, magic that would determine if you are telling the truth indicates you are being truthful if you so choose, and you can’t be compelled to tell the truth by magic.
Insightful Manipulator
Starting at 13th level, your ability to read the people around you is heightened. When you make a Wisdom (Insight) check to assess the personality and mannerisms of a creature, you may make the check with advantage.
Additionally, you may assess a creature by spending at least 1 hour outside of combat talking and interacting with them. You gain a bonus to attack rolls or contested skill checks made against them equal to your Charisma modifier (minimum 1). Only one creature may be assessed in this way at a time, and assessing a new creature causes you to lose the bonuses on the previous one.
Grand Strategist
At 17th level, your plans and strategems become so intricate that they interlap with your normal combat abilities. You may ready an action in addition to your regular action.
You can use this feature a number of times equal to your Charisma modifier (minimum of once), and you regain all expended uses of it when you finish a long rest.