Still doesn't matter for the most part, immunity or not.
1) Delay Poison, once available to have on all day, removes poisons from consideration. For Poisons to matter, powerful poisons would need to be classified with curses eventually.
2) Poison DCs scale wildly. This is more an issue with supply. As is, the table of poisons available does not properly describe a range of poisons suitable to a poison oriented character.
3) After immunity to poison, most poisons simply inflict ability damage. Immunity is common enough and the effect is either hyperlethal(targeted at a weak ability) or ineffectual(targeted at a moderate ability thats not its attack stat). Variety of effects is advised.
4) Intervals. Initial damage and secondary damage means that what with the poison kicking in after a minute, most poisons are a) pretty obvious, b) too slow to use in combat, c) second saves tends to wipe the secondary anyway.
5) Streamlining. Poisons, curses, and diseases are all Effect Over Time things, why have each run on its own subsystem?
6) Crafting poison takes forever due to how the craft rules work.
1 is simple enough, its just a consequence of the immunity rules. Either you throw in poison-like effects that aren't poison, or you revamp immunities.
2 & 3 are matters of variety. Expand the types of poison. Vary what they do. For the stronger variations of weaker poisons, you could simply count the weaker poison as the raw material for the stronger's manufacture.
4 & 5 can be solved with the Affliction
mechanic from Pathfinder(100% compatible with classic 3.5), combined with an expansion of venom variation(a long onset poison might work for assassination for example, an hour onset poison that causes fatigue, followed by unconsciosness and Con loss).
6 the answer and the problem is both Minor Creation. With it, expensive poisons are trivially easy to purchase, without, forget crafting your own, it takes weeks to make enough poison that you'd blow through in a single day. Or you could fix the craft rules.