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Legend
A couple of other ways I have thought of:
Substitution levels: Discrete abilities that represent a concept which replace an ability of X level or higher. This could mean that a wizard who chooses the illusionist tradition would have all of the illusionist tradition abilities, but at 6th level he gained the Ninja base subclass abilities instead of the 6th level illusionist ability.
Feat chains: Every class gains ASIs at the same rate (with the fighter and rogue gaining a couple extras) so it could be a good place add general subclass abilities. The problem would be balancing them with the concept and the power level of a feat. It also cuts into a player's ASIs which may not be preferable, and as always, not all games use feats.
Substitution levels: Discrete abilities that represent a concept which replace an ability of X level or higher. This could mean that a wizard who chooses the illusionist tradition would have all of the illusionist tradition abilities, but at 6th level he gained the Ninja base subclass abilities instead of the 6th level illusionist ability.
Feat chains: Every class gains ASIs at the same rate (with the fighter and rogue gaining a couple extras) so it could be a good place add general subclass abilities. The problem would be balancing them with the concept and the power level of a feat. It also cuts into a player's ASIs which may not be preferable, and as always, not all games use feats.