LogFAQs > #971178268

The concept of sub-classes is already there in the form of divergent evolutions (Eevee being the most dramatic example of this), and anything a skill tree would do (active abilities, passive abilities, stat boosts, from which you can only select a few options) is already covered by other customization mechanics already in the games (move choices, abilities, held items, EV training). As a concept, adding more specialization options won't necessarily make more Pokemon viable, unless those options are unique buffs to currently-nonviable Pokemon (in which case, buffing them directly would do the job).

papercup posted...

I wish they would do more things like Dhelmise where it effectively has 3 STABs.

Tera types basically present the option to do this for everyone, though with a lot of caveats. There are also quite a few moves in Gen 9 that mix up type relationships, like Freeze Dry being SE against water types despite being an Ice move.