Horwath
Legend
3.5e fighter and wizard were designed that way 100%.Fixed feat slots that reinforce the class fantasy and refine it into a more specific archetype, hence, subclass. Portable "subclasses" that are designed to work with multiple different classes won't reinforce and refine the base class fantasy--quite the opposite.
Now, portable subclasses are not a terrible idea in and of themselves. There are probably games already that use them, calling them templates or some such thing. But I feel that they would pull D&D toward being a different sort of game and rob it of one of its strengths.
you only had option to pick whatever you want.