After character generation, in the HallOfSelection, the player won't choose the class for the character, but the class guild to join. The difference is, you're able to leave a guild to join another one, to learn a new profession.
Changing the guild is not as easy as it sounds. The character has to fulfill the guild rules. For example members of the warrior guild have to make sure that the fighting skills are much higher than all others. A warrior having a higher skill in magic than fighting won't be accepted or even become ostracized.
Guilds will have a bunch of skills they teach. Without a teacher you never ever get a chance to advance over amateur grade of the skill (lowest capability value of 1.0 - see skills). So you have to be member of the guild to reach the master grades. The choice of the class profession will limit the capability but not the skill level. So everybody will still be able to reach the maximum skill level in every skill. It just takes much longer for all but the “guild skills”.
The guild skills will advance every [overall] level as long as you got a few xp in them. If the top 5 skills (where the character got most xp between two levels) aren't guild skills, but there are also xp gains in guild skills, more than 5 skills will be advanced. That's because the guild masters are able to teach the skills and offers deeper knowlegde.
If the character just uses one guild skill, only this one skill will advance. Without using a skill / training no learn effect at all.
This can't be abused, because if a fighter kills just one monster each with one handed weapons, two handed weapons, missile weapons and karate and gaining most xp with magic, than the fighter will become ostracized of the guild after a few levels, because the fighter no longer follows the guild rules.
The guild rules for the warrior guild could look like this. The skills missile weapons, one handed weapons and two handed weapons has to be at least twice as high as magic skills. Also at least one of clawing, flame touch and / or karate (depending on the race) has to be twice as high as the magic skills.
For example a warrior with
|one handed weapons||12|
|two handed weapons||11|
will be rejected to enter the guild and getting advantages over the guild if at least one of the magic skills evocation, praying, pyromancy, sorcery, summoning or use magic item is 4 or higher.
Alchemy, inscription, literacy, sense curse, sense magic and probalby more or all other skills may be added, to make a well-defined distinction between the guilds.
This needs to be balanced between all possible class guilds. It wasn't discussed in that detail.
Good class items are also available through the guild. For example the pyromancer will be able to get meteor swarm once the level is high enough and the quest is solved for it. No other than the pyromancer as a member of the pyromancer guild will get access to this powerful spell.
The guild will offer an “apartment” for the player. A uniq room for the character to place stuff. This room will also be extentable after solving some quests for the guild. For example teleporters to other regions, a magic box (pocket reality) like in the extended scorn apartment, …
This extensions of the “apartment” (should be called a room if it's part the guild) are fixed to the guild the character solved the quests for. Don't make the rewards like teleporters moveable it the character changes the guild. Betraying the way of living of the guild should be a punishment. Enforce more role play gaming.
There is also an introduction part in the guild to show new players how to play.