I would consider this a discussion for the new PCRPG.
I don't think class should be something decided when you first start out. It's something you should decide over time once you've tried various skills to see what you like. Once certain core skills (weapon/casting skills) have reached a specific level, you'd be given the option to train into a class. If you accept that as your class, your improvement rate for those class-specific skills can increase, say 1.5x the normal skill improvement rate. You also learn new class-only skills.
If you refuse that class and want to try something different, you skill improvement rates stay the same until another skill hits that level.
Now the question is whether you allow players to specialize in more than one class. If they master a class, can they go back and raise other skills to join in another class? If they do, do they forget all that they learned from their original class?
Certainly they should be allowed to change classes, but the main question is what can they keep and what do they lose for doing it?
Maybe each successive class that you learn decreases the improvement rate for the other skills that you haven't specialized in, yet.
A person who specialized as a fighter with Slashing and Endurance then dropped his class would learn casting skills at a lower rate than originally (say, at 0.8 of the original rate). Once his skills hit the class decision level, he can specialize as a mage and learn mage skills at a rate of 1.5 of the 0.8.
After he's learned the mage skills, he can drop the mage class (or his class just becomes modified. He's now a Casting Fighter) and learn thief skills like stealing. But these only increase at a rate of 0.5 since he's learned two classes already. Once he joins the Theif class to train, it's 1.5 of 0.5 and he becomes a Mugging Berserk Mage...or whatever.
This increases the amount of time it takes players to master all classes and become a god, and still allows them to continually increase skills so the game does not become stagnant.
Each class should have specific skills that you can only access when you become a member of that class. You should learn those skills, over time, say one every couple of levels. When you go to change classes, it might be cool if you can keep one of those special skills that you can use even though you drop that class and become a hybrid player. This allows for players who want to be a single pure class to have a reason for doing so..they can get some high level skills.
You shouldn't be allowed to take with you just any skill. Players would train to get the 'best' skill then drop the class. Perhaps class-specific skills can be set in tiers and you can only keep a first or second tiered skill.
A specific example to help better illustrate..
A player starts out fresh. He decides he kind of likes the casting skills, so he focuses on killing enemies with low level spells, maybe a piercing weapon. His skills increase at a rate of 1.0 each level. When he gets to skill-level 10 (we're making up levels here, it really depends on experience rates and whatnot), a Bot in town tells him that he might be interested in the Guild of Mages. Talking to a bot of the Guild, he is invited into the Mage guild and becomes an Apprentice Mage. Joining the guild he has learned a new spell, a more powerful Fire spell, the Firey Flamer. Over time, his level and Guild Reputation (I'm bringing in reputation because I like reputation systems..) increase. Your Guild Reputation decides what Class-Specific skills you can learn (like Firey Flamer). These are the Guild Secret skills that only members can learn.
The player continues leveling, using new skills as he earns them from his guild, increasing power in his original skills (which now improve at a rate of 1.5. His other skills like slashing, lock picking, now only go up at a rate of 0.
. Eventually he becomes reputed enough within his Guild to learn their Ultimate skill, Super Mega Ice Death. But by now he has become tired of just being a mage, so he leaves the guild and starts to train his lock picking. He loses the Super Mega Ice Death spell, but retains knowledge of Firey Flamer. When his lockpicking reaches skill-level 10 (at 0.8 per level), he is invited into the Thief Guild. Upon acceptance he becomes a Thieving Mage and his reputation with the Mage Guild decreases as his Thief reputation increases.
Players who change their mind can simply drop their new guild and go back to their original guild without having lost too much of their work.
His thief skills (lock picking) now increase at 1.5 x 0.8, and his mage skills drop to the levels of non-thief skills such as slashing (now only 0.5). He decides he really only wanted to have Steal, the first tier Thief skill, so he leaves the Theif guild. He feels he'd like to be able to bash players, so he begins the process of training bash and is invited to the Cleric Guild. Bash now increases at 1.5 x 0.5 and mage/thief/fighter skills increase at 0.3.
And so on...
