If this is truly what you are trying for... then I think what you are going to have to do is build one "blank" generic class. Figure out the different feature slots for each level, and then put every single game and class feature and every spell into one big selection pot. This is the only way you are going to be able to balance all the features against each other, while simultaneously determining the level prerequisite for each feature as well. Basically you need to Skills And Powers your game.
Once you do that... get all features, spells, and selections created and their costs determined... can you then be able to build your standard classes out of the system. You won't ever need to multiclass ever again, because you can select the features you want at every level, some of which will match the "class builds" you will supply with the book (for those people who don't wish to create their own build for their character. )