I think you'd have to do something like 3e and 5e have done and rate abilities numerically and then give PCs X points at 1st level to buy abilities. So if you want heavy armor, semi-spellcasting, sneak attack, and bardic inspiration, you would spend your points on those things. You'd also gate abilities not just by point cost, but the levels(the lowest level if different among multiple classes) at which they are offered. So when you hit 3rd level, you could purchase from among the 1st-3rd level class/subclass abilities. You might also create chains in which you need a lower form of an ability before you can get the higher one. As an example, you could not buy extra attack(2) at 11th level unless you had purchased extra attack already.