Thing is, I'd like to get away from mechanical customization and put a lot of it into sheer flavour and personality. You can have six characters running on exactly the same mechanical chassis (e.g. six basic Fighters) and yet each will be vastly different from the others in play simply due to their role-played quirks, personalities, language, etc. That's what I'm after.
Personally, what you are describring that is why I and many other gamers I know (and knew) left D&D for other games before returning with 3e. I want the mechanical customization. Idon't care about optmizing for combat or power. I do want the mechanics to reflect what my character is good and is not good at and having the opportunity to pick things up new things
Poisoner, for example, should be a baked-in class feature for Assassins and maybe some specific Cleric types, and not available to anyone else. (i.e. anyone can try using poisons but those trained as Poisoners are much better at it)
Linguist sounds like a background; i.e. something you only have to choose (or roll) once and may or may not have any mechanical effect beonyd perhaps - again only once, at char-gen - giving you knowledge of an extra language.
Why shouldn't my fighter, ranger, rogue, druid, bard, or whatever have access to Poisoner? Maybe, they grew up in a culture that used poisons for hunting and/or defense against agressive outsiders. Maybe, along the journey, the group stayed with a people who do use poison for those purposes? Maybe the character apprenticed with an herbalist, alchemist, or assassin who taught them about poisons before events led them to study the class in which they begin?
Observant? If you mean alertness, i.e. harder to surprise, 1e baked that in as a Ranger feature and that always seemed like a good place for it, though a solid case could be made for baking it into Rogues and maybe Bards as well. In any case it's easy to lock in as a class feature. If by 'observant' you mean something else, please elaborate.
Why shouldn't a fighter, who served as bodyguard or sentinel or whom grew up among hunting in a rural environment or among hunting nomads have alertness?
Why shouldn't a barbarian have keen senses from growing up in a non-urban environment, growing up among hunters, in a situation in which he could be attacked at any moment, or being raised by animals?
Why can't a monk have alertness from zanshin which is " a state of awarness or relaxed alertness" and something associated with several martial arts?