Actually, this prestige class concept was one I wrote for FFE's Encyclopedia of Prestige Classes. It is along the same lines but without a spell capability. See how you like this one...
Mimic
There are a few gifted begins in every society that have a knack for learning things quickly. They are neither instructed nor pressured to learn them, but rather do so instinctively. The mimic is representative of these people and has the ability to study and duplicate actions that most others would require years of patience to do. His life is one of constant discovery. By discontinuing (at least temporarily) the focus of other classes, he is free to adapt to any environment.
While this talent to accurately mimic skills and abilities is a great gift, it also supports a wholly subordinate mindset. Mimics are never leaders, their actions and even thoughts based on how they have observed other beings act in similar situations. Those who pursue this class depend as much on their personality as they do their intellect. But as noted above, a mimic’s ability comes naturally and therefore this prestige class is no more an extension of one class than it is another.
In one aspect, mimics are loners. They do not form collectives of their own kind. In fact, it is rare that one of this class even meets another. On the other hand, mimics are always found within some kind of group; be it a city, town, or even an adventuring band. Just as a mirror needs someone to throw a reflection, a mimic needs others from which to draw traits to emulate. In an urban setting, mimics can be found as errand runners, journalists, sellswords, scribes, teamsters, and even barkeeps. Essentially, any vocation that brings the mimic in contact with multiple persons and/or places is preferred by the class. Mimics generally do not hide their gifts and those people who know such characters appreciate them as a carpenter does a complete tool set. For what these characters are not trained in they can at least give a solid effort to.
Evilly-aligned mimics are another matter. While they still keep to a community, they are usually doing so to spy for a criminal organization. These characters present a threat much like a doppelganger in that they study a person for the purpose of eventually eliminating and replacing them. Particularly skilled mimics have even been known to replace thieving guild masters and barbarian tribesmen for a great length of time before discovery.
Hit Die: d6
Requirements
To qualify to become a mimic, the character must fulfill all the following criteria:
Attributes: Charisma 15+, Intelligence 15+
Base Attack Bonus: 4+
Skills: Listen 8 ranks, Spot 8 ranks
Feats: Alertness, Combat Reflexes, Expertise, Skill Focus (Concentration)
Class Skills
The mimic’s class skills (and the key ability for each skill) are: Balance (Dex), Bluff
(Cha), Concentration (Con), Disguise (Cha), Gather Information (Cha), Knowledge (any)
(Int), Listen (Wis), Search (Int), Sense Motive (Wis), and Spot (Wis). The mimic can attempt any other skills untrained with the condition that he be around someone who has used the skill for a short time (see “Bonus Class Skill” below for further explanation on this condition). See Chapter 4: Skills in the Player’s Handbook for skill descriptions.
Skill Points at Each Level: 8 + Int modifier.
Class Features
All of the following are class features of the mimic prestige class.
Weapon and Armor Proficiency: Mimics gain no initial proficiency in weapons or Armor (see the Bonus Proficiency ability below). Note that armor check penalties apply to the skills Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Slight of Hand, and Tumble.
Bonus Class Skill (Ex): Beginning at 1st level, the mimic may add any skill to his list of class skills. This skill can even be exclusive to another class, but it must be one that the mimic has regularly witnessed being performed. It is the GM’s discretion as to whether a mimic has been appropriately exposed to the desired skill. As a general rule, the skill should be granted if a fellow adventurer in the mimic’s party uses it regularly (once every adventure, for example). For each level gained as a mimic, the character adds another class skill in this way.
Bonus Proficiency (Ex): By bearing witness to another’s use of arms and armor, the mimic can permanently add one weapon or one armor to his list of proficiency feats. No item is forbidden (even exotic weapons) so long as the mimic has had the chance to see it used on a regular basis (as above, this is at the GM’s discretion). The class grants this ability at 2nd level and at every other level after (4th, 6th, 8th, and 10th).
Mimic Class Ability (Ex or Su): Any extraordinary or supernatural ability of another class can be used as if the mimic’s level was that class’ level. This means that a 4th-level mimic can try to emulate the special ability of another class at or under that level, but not over 4th level. For example, this mimic can attempt to emulate a rogue’s sneak attack at +2d6 damage (a 3rd level power) but not a sneak attack at +3d6 damage (a 5th level power). Mimicking a class ability is a move-equivalent action and requires a Concentration check (DC 20 + class level being mimicked) for each use. An ability that is used for more than one round forces the mimic to re-roll the Concentration check each round. Any special prerequisites of the ability must be met by the mimic (such as appropriate ranks in Perform for bards). As with gaining a bonus class skill, the ability mimicking can only be attempted for those powers seen in use more than a few times (at the GM’s discretion). Of course, the mimic has normal access to those special abilities he possesses from previously taken classes and need make no special roll to use them.
Mimic Opponent (Ex): Beginning at 3rd level, the mimic can quickly assess the fighting technique of his opponent. After engaging in melee with the chosen combatant for one round, the mimic gains a +1 competence bonus to both his AC and attack rolls against that foe. This bonus increases at 5th, 7th, and 9th level and remains in effect as long as that foe remains the mimic’s target. This special ability can not be used at the same time as the Mimic Class Ability (see above). The concentration required for one offsets the ability to perform the other simultaneously.
Mimic
Base
Class Attack Fort Ref Will
Level Bonus Save Save Save Special
1st +0 +0 +2 +2 Bonus class skill, mimic class ability
2nd +1 +1 +3 +3 Bonus proficiency
3rd +2 +1 +3 +3 Mimic opponent +1
4th +3 +2 +4 +4 Bonus proficiency
5th +3 +2 +4 +4 Mimic opponent +2
6th +4 +3 +5 +5 Bonus proficiency
7th +5 +3 +5 +5 Mimic opponent +3
8th +6 +4 +6 +6 Bonus proficiency
9th +6 +4 +6 +6 Mimic opponent +4
10th +7 +5 +7 +7 Bonus proficiency