Talent Cube

Imagine a typical MMO "talent tree": several possible specs, each divided into several tiers of useful abilities, from least-useful to most useful.

Imagine instead a cube.  Each face of the cube is subdivided, like a Rubik's cube, into 3x3 or 5x5 or whatever smaller squares.  Each of these squares is a talent.  At the center of a cube's face is the core talent that makes a given class what it is: fighters hit hard with melee weapons, mages cast spells, whatever.  If you have this talent, you can do the thing which distinguishes a class.

Surrounding this are the second-tier talents - the things which amplify your ability to do it in some generic fashion.  And surrounding those are the third-tier talents, ones which are interesting to your spec but not vital. And so on.  For our example we'll imagine a 5x5 cube.

Your "career path" starts from a given square on any face of the cube - the one you initially chose as your class.  Every level, or every time you receive a talent point, you can advance to a square adjacent to one you've already taken.  Thus, you move from your first-tier core talent into any one of eight second-tier talents, and from there either around the loop of the other second tiers or out into the third tier.

You may also wrap around the cube.  For example, I can start on the Fighter face, move left to the edge, and advance from the edge into the heart of Cleric.  I'll get presumably useful but not core cleric abilities as I go, and I'll approach the core cleric ability ("cast holy spells").  It makes me less of a fighter, but more of a cleric than a pure fighter could be.

A 5x5 cube will have 1 core competency, 8 second-tier abilities, and 16 third-tier abilities.  So in 25 steps I can be a fully capable member of any given class, and in 20 steps I can have basic competency in 4 classes.  It takes 38 steps to be basically capable in all six classes which this system permits, and 120 to have every possible talent.

With an MMO level cap of 60, we could give out a talent point every 2 levels after 10, letting you "complete" your class at 60 or be halfway good at two classes at once.

Likely classes in this system are Fighter (melee weapons), Mage (destructive magic), Cleric (healing magic), Thief (assassination), Ranger (ranged weapons) and X (unknown).  Arrange classes on the cube to be ideologically opposite, so Fighter-Mage, Ranger-Cleric, and Thief-X.  This makes Fighter adjacent to Cleric, Ranger, Thief and X.  Fighter/Cleric combos should be good paladins, etc.