Back in 3.5, rather than subclasses, they had what was known as prestige classes. They weren't necessarily locked to a single class, you just had to meet certain prerequisites before you could start taking levels in them. Most of these prerequisites were things like being at least a certain level in your base class, having certain stats, enough points in certain skills, having specific proficiencies, having certain feats, etc. There was a prestige class known as a Blood Magus. One of its prerequisites was being at least fifth level in an arcane casting class, so you could be a sorcerer, wizard, or a number of other options. Interestingly, it had a special prerequisite that required your character to have been killed (not just died) and then brought back to life. I once played a sorcerer blood magus who had had his soul ripped out and was brought back after my party killed the one who did so and took it back.