I'd use the following for water vulnerability...
* 1d6 points of damage by a mundane water attack (having a bucket of water tossed at it).
* 5d6 points of damage per round of being submerged in water (bull-rushed into a river).
* 1d6 points of damage per spell level by a water-based spell (wall of water).
That sounds good. I reduced it to 1d4 so you couldn't douse a Small elemental with a single bucket of water.
For fire-based spells, I'd give them one per 'age category' And probably 3 times per day...
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You might need to be careful about this, because a summon monster to get a fire elemental might become much more useful by doing this...
Yeah, there is that. And, fire elementals would be more powerful than the others unless they ALL get SLAs.
For my own fire elementals (as well as air and water elementals), I've given them an ability called insubstantial which grants benefits similar to the gaseous form spell, as well as DR 5/-.
That's pretty close to what I have for immaterial - it's a 25% miss chance and chance to ignore physical attacks, but you can't pass through solid objects. Elementals already gain DR x/-... how does that work with your system? Does that just replace the existing DR?
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Okay, so I thought of something a little bit ago - a modification to the Burn ability. If you set an item on fire, it'll burn, but unless it's either really small or really flammable, it'll go out after a minute or two. So what I was thinking is this:
A fire elemental can set objects alight with its touch. Each round, the object takes a variable amount of damage, depending on the elemental's size, and it burns for 2d4 rounds. (The reasoning behind this is that the older and larger it is, the hotter its flame - hotter flames deal more damage.) If an object's hit points are reduced to 0 in this manner, it is destroyed, burned beyond repair. It can be extinguished normally (by casting a cold spell on it, dousing it in water, etc.)
A fire elemental can also deal ongoing fire damage to an object by maintaining contact. This is often used for nonflammable objects, like metal. Each round, the elemental deals damage according to its size (1d6 for Small-Medium, 2d4 for Large-Huge, and 2d6 for Greater-Elder). If the object is flammable, it continues to burn as noted above if the elemental ceases contact. If the object is metal, it is treated as being affected by a
heat metal spell for 1 round per Hit Die of the elemental.