The existing mephits have been tied strongly to the elemental, quasi elemental and para elemental planes. That means there is a lot of room for expansion and I have come up with 8 new types, ones that go beyond the elemental vibe for the most part. Instead of coming up with all new stat blocks (which most of you will need to adapt anyways), I have just included their breath weapon, immunity and general description.

Chemical

Breath Weapon- weak poison (5/15 in LL AEC terms)

Immunity- acids, poisons and other chemical attacks

Chemistry is different from alchemy. The latter is transmutation of matter and soul. Chemistry is the study of matter and its possible uses. Chemical mephits look like glass filled with flowing colors that change from moment to moment. They are just as dangerous as other mephits as their spit is a poison that can affect any creature, even those that are normally immune to poisons. The poison seems to be corrosive, but it is much more complex than that- it warps molecular bonds rather than breaking them. With enough time and effort a chemical mephit can destroy any mundane object or creature. Magical items and creature protected by magic are usually immune to this spit. The mephits heal when dipped into a container of chemicals (which can include a stomach or other body part).

Fecund Growth

Breath Weapon- tumors

Immunity- druid spells, plant and animal based magic below 5th level

Similar to chaos beasts, fecund growth mephits represent life at its worse, yet they can be the source of new species. Where chemical mephits affect all molecular bonds, fecund mephits warp genetics. They look like tumorous mushrooms and have no wings (nor can they fly). Their breath weapon causes cancerous tumors. Instead of killing months or years later, it only inflicts a small amount of damage but that damage can not be healed by spells weaker than Heal. Creatures that die from this breath weapon have a 1% chance of being reincarnated as per that spell (though I suggest a new chart with all the creature types). They heal when targeted by polymorphing type spells.

Gilded

Breath Weapon- damages gear

Immunity- illusions

Gilded mephits represent the false nature of poorly made, yet expensive, goods. They wear clothing and appear to be wealthy. Their breath weapon turns real gear into fakes. Damage, armor class and such suffer. Material components become useless. Coins and other wealth turn to lead or other base metals that still appear to be genuine until tested. Gilded mephits are summoned by those who wish to destroy their wealthy enemies by destroying them economically as well as weaken their defenses. As they are masters of deception, gilded mephits can see through illusions with ease. In fact shadow energy heals them.

Light

Breath Weapon- blinding and heat

Immunity- light and dark based magic below 5th level

Light mephits represent the worst aspects of sunlight- blindness and drought. They are difficult to look upon and may blind those near them without needing to use their breath weapons. That breath weapon not only blinds opponents, but also dehydrates them. Weak undead burn near, and may even be destroyed by, a light mephit. People who summon them to do just that usually regret it as the mephits rather rampage and destroy crops and other vegetation than fight zombies and ghouls. The mephits dispel all weak darkness spells they come into contact with and heal from those based on light.

Noise

Breath Weapon- sonic damage

Immunity- sounds and silence effects below 5th level

The enemy of bards and music lovers, noise mephits look like air mephits but with ripples that randomly travel over their bodies. It is impossible to be suprised by a noise mephit, even for deaf people- they make the air vibrate in the most annoying ways. They heal from magical and mundane sources of loud sounds and dispel silence magic below 5th level.

Numbers

Breath Weapon- confusion

Immunity- law and chaos spells below 5th level

The most ordered of mephits, these creatures use numbers to sew chaos, ironically. They appear to be boxy robots with very bland features. Their breath weapon causes those who fail their save to become so lawful they can not deal with the normally chaotic world. Their mind goes into a tail spin and are effectively confused for a short time (when it wears off). They are immune to chaos based magic below 5th level and heal when affected by any lawful spell. I was considering allowing them to alter numbers, such as mass, distance and time, but those are much more powerful than what a mere mephit should be able to do.

Shadow

Breath Weapon- shadows

Immunity- light, shadow and darkness spells below 5th level

Shadow mephits are similar to shades in that they become weaker in bright light and complete darkness. They are black with glowing eyes and four flickering shadows, each in a different direction. The breath weapon covers their targets with twisting shadows that make sight difficult with the constant changing of light intensity (penalties to all actions that require seeing). They dispel light and darkness spells below 5th level they come in contact with and heal when targeted by shadow spells.

Wood

Breath Weapon- entangle

Immunity- druid spells (except those based on fire and ice) below 5th level

The only elemental mephit of this list, wood mephits look like animated vines with arms and legs. They love causing vegetation to move and warp (shape wood at will) and can do this even with dead plants. Their "jokes" usually involve bringing down log homes or wooden bridges. Their breath weapon is an entangle spell and they heal from any plant based spell, even those that kill or weaken plants. Unlike many mephits, wood mephits do have a weakness- cold and fire based attacks do double damage to them.