This is something of an open question and solicitation of opinions.
A very common thing in older editions of D&D is specific spell lists for certain classes. Rangers and Paladins are prime examples. One thing I never really put too much thought into is the limitations of such a list. Does it mean that a spell casting class can only ever use spells on its class list? Maybe the spells on the list are the only ones that can be selected from freely, like when a level is gained. If that is the case, what about researching new spells? Could these be any spell the player wants?
I kind of think they should be limited to only what's on their lists, since a limited list seems to be a balance thing. But, it doesn't make much sense, really, from the character's perspective. If there was some sort of college of magic situation, then it makes more sense. Or maybe a source-of-power sort of thing.
This is pertinent because I am working on a spell using class for Delving Deeper. I want a limited spell list to be one of the balancing factors. So, from a DM perspective I wouldn't want a player of the class to go trying to research Fireball, which isn't on the list for the class. On the other hand, I can understand a player finding a scroll with Fireball on it and saying "I want to copy it into my spellbook", and being pissed if I said no, with no game-world reason for it.
I know this is post is a bit more disjointed than usual. I hope I made enough sense to get my question across so I can get some advice.