So my roommates and I have started a tabletop game using some online rules that quite simply try to put a Final Fantasy flavored candy coating over the Pathfinder/3.5 rules; located at this site if you are interested in taking a look yourself. In an effort to include some of the class customization in the series, be it by the job system, dress spheres, or what have you, I am implementing an artifact I am, for lack of a better name, calling a Crystal of Heroes.

Upon aqcuiring a crystal, the bearer will have the abilities of one additional class unlocked and available to him. Only class abilities though, no change to hd, hp, saves, etc. When exp is rewarded they can then choose to invest some/all of their exp into the crystal in order to unlock more classes, or advance the abilities of an already unlocked class.

As an example, on of the PCs is a Ninja, when getting the Crystal she will gain access to the class features of a 1st level Black Mage. Meaning she gains cantrips, 1st level spells, and MP as if she had taken a level in BM but no increase to HD, saves, skill points, etc.

I haven't decided how much exp I want to require the PCs to invest into their Crystals to unlock new classes or level previously unlocked classes. So my questions are; 1, how badly am I going to end up screwing myself over as GM by allowing something like this? Does making them choose between gaining character levels or unlocking more options provide enough balance? And how much exp would you suggest I require to unlock new classes or advance unlocked ones?

You could end up with a superhero character with loads of classes if you give them out too freely. I would make them very hard to get. Increase the XP required to unlock higher levels and gain access to additional classes. For example 5000 XP to open the additional black mage class at level 1. Then maybe the regular required XP to gain each additional level. Second unlocked class would be at 2x the normal cost and 3rd class would be at 3x normal cost.

I was thinking an initial xp investment of roughly 250 to unlock the first level of a new class, and then from there half of what it would cost to gain a level in the class, and I say half because the pc is not gaining hd, saves, skills, etc from the levels gained in the crystal. They would be slowing their own progression by investing in the crystal to unlock new options.

So using the Ninja I mentioned in the first post again. When the crystal is initially obtained it will unlock Black Mage abilities for her. To gain the next level of Black Mage abilities she would have to invest a total of 650 xp into the crystal, which is then 650 less xp going toward becoming a 2nd level character. If she wanted to gain access to White Magic, either through unlocking White Mage or even Red Mage, she would have to invest 250 into the crystal initially to open up that class at first level, again meaning 250 less xp being used to increase character level at the expense of gaining access to new spells that would still consume MP normally, and as Ninjas don't usually have a MP pool that could prove troublesome.