My logic was that unless there are rules to modify the text of an ability such as Venom Immunity, then the rules are the rules. The rule for druids is they are granted immunity to all poisons. The rules for oozes are they're immune to poison, but it's possible to create poisons that affect them anyway. My logic is that because the druid has no such modifying text, it is immune to all poisons no matter what.
Your logic is flawed in that you refuse to acknowledge that the section is only covering
creature immunity. Because it fails to mention any thing other than
creatures with immunity, you cannot apply the logic to character granted immunity. Your rationale for analyzing this section is self-contradictory. I'm going to quote the section once again and you show me where character immunity is mentioned:
Creatures with natural poison attacks are immune to their own poison. Nonliving creatures (constructs and undead) and creatures without metabolisms (such as elementals) are always immune to poison. Oozes, plants, and certain kinds of outsiders are also immune to poison, although conceivably special poisons could be concocted specifically to harm them.
The section starts out talking about CREATURES and continues to talk about them throughout. Trying to cherry pick the last sentence and say that, "Oh, since Druids aren't mentioned, it clearly doesn't apply to them" fails to grasp the fact that the entire section isn't a statement for or against Druid immunity. In fact, because WotC makes a distinction based on the nature of the immunity i.e. metabolism versus none, Druids clearly fall in the "I have a metabolism" category and are thus more likely to fall under the "special" poison exception. That would be a logical reading.
The thing is, those poisons on the list are the only ones that have been given rules for in that book.
Hallelujah! That's right. The list of poisons is what they mean when they say "all" poisons. The list you said didn't exist, does in fact exist which and none of them are magical and none Of them are supernatural or spell-like. Which is why "all" would not refer to magical or supernatural poisons.
If something is immune to all poisons from that list, they're effectively immune to all poisons because those are the only poisons.
Praise be to god. This is the logic I am employing when I offer the interpretation that a Druid is not immune to magical poisons (ignoring that previous versions of D&D explicitly excluded magical poisons). D&D doesn't recognize any magical poisons on its list of poisons. Why it fails to address spell based poisons directly, and then adds specific text to Cloudkill is confusing.
If one were to make up a new poison, then that falls squarely inside house rules because that's creating something that does not exist within the context of the official rules.
Now you're trying to be sneaky and argue that any new poison is a Rule 0. I'm not going to debate whether adding a "special" poison is a house rule. It doesn't matter. The rules say such things can exist so doing so is within the bounds of the rules. What's relevant to this discussion and the OP's question is whether the Druid would be immune to this "special" poison that was magical or supernatural. The answer, based on the fact that all the known poisons are non-magical, is no. Why? Because as you have finally acknowledged, there is a list of poisons,these are "all" the poisons the game is referring to, and none of them is magical.
Yes, such a clarification was necessary in this case. Why? Because cloudkill is technically a spell, and rather unique one at that. As it's definitely magical and not a standard poison, clarification would be needed to determine whether poison immunity would work on it.
BINGO! Ergo, poison immunity does not automatically include magical poisons. And we've already established that the "all" refers to the list of poisons of which this spell is not on it. Could you argue that "all poisons" would have encompassed this? Yes, but it's not clear. Just as it's not clear that there is a difference between Construct immunity and Ooze immunity.
If we're going strictly by the book, no, because those things aren't mentioned in this exception.
Flawed reasoning, see above.
The catch is, there are no official rules for these "special poisons." Everything beyond "conceivably special poisons could be concocted specifically to harm them" is left up to players to figure out for their own ends. Because of this, by the book, druids (and everything else except maybe for oozes, plants, and certain kinds of outsiders) are immune to all poisons.
Once again, that doesn't follow. The section you keep referring to applies to creatures, not character immunity. It isn't trying to make a statement on character powers one way or the other. And any objective reader is going to reason that things with a "metabolism" are most likely susceptible to "special" poisons, which puts Druids clearly in that bucket.