I thought of this once.

We have fiends and undead, which resist shadow and deal shadow. But we could have undead that deal elemental+resist shadow etc.

But then I had another thought.

The devs could just make a 5th damage type if they wanted to. Infinite monster families!
But maybe it isn't for the best of the game? Perhaps only 6 monster families is for the better?

Interesting. Many people knew this or thought about it (I didn't think it was thread worthy so I didn't.) I'll list some ideas, but Nick is gone, so his rule doesn't really mean anything anymore. I think it is obvious they won't do this new class thing but it is worth a shot.

Familiars. They disguise their look and attacks to be like random enemies, but they always deal shadow damage.They turn into their normal look in their dying animation(So they attack in different was and with as much damage the enemy they are disguised as would do to you, but in shadow damage.)

Drake. These guys have a similiar AI to the snarbolax. He tracks you before he bites, he moves towards you like the snarbolax and he roars. He can temporarily fly but not far. They can charge at you, dealing heavy damage, swipe their tail at you, bite you, scratch, and breath a tiny bit of fire. (Kinda like a slag but lasts a lot shorter)

Dragon These guys are the same thing as drake's but they fly up and swoop down sometimes(this damage enemies too.), their fire breath goes twice the length as a slags breath but lasts shorter and is more of a thin line. (animation looks a lot different too.) When he does any physical attack it has a 40% chance to deal fire or stun. Fire makes him deal more damage, and he can only appear once in a level (This doesn't apply to danger missions or future events)

Some ideas are:
Almiranian Knights:
A Kat Konjures up a portal that creates a paradox in the space time continuum and brings back the Almiranian knights, and we have to fight them. They would fight like us and maybe we could introduce new weapons with them. (Owlites might come back too and maybe we'll get a new ally.)
They are weak to shadow, resistant to elemental and deals piercing.
Dragon and drake could go under them. (They had pet dragons. Don't judge.)

Elementals:
A new planet crashes into Cradle, and it consists of these elemental based creatures that are can take any shape or form but are covered in their assigned element. They are extreemly hostile.
They are resistant to elemental (Duh.) weak to piercing and deal elemental.

Apocreans:
The Apocreans have come back, because Aprocrean Harvesters sacrificed all of their dark energy to bring back Apocrean knights that resemble the apocrean harvesters and have similiar attacks.
They are resistant to Piercing (Piercing doesn't even hurt them), weak to elemental and deal Shadow.
The Familiars can go under this.

Apocrean knights would work too. They are super resistant to piercing. (They don't even take damage) Apocrean knights materialize in a different place super quickly, and their defense goes up if you try to hit them with piercing. They attack like Phantoms, but instead of charging their weapons, they switch to their guns. They can extend parts of their body to places giving them long reach(They are pretty much made purely off of several tentacles, a brain and spikes.) They can extend their hands to a long reach, which can immobilize you, while they damage you,(You have to attack his arm to cut off his grip) , pull themselves to you then swipe with their sword or damage and poison you when they stab you from afar. They can stab their sword into the ground creating a small shockwave that makes hands immobilize, damage or poison you. They should also be able to materialize into the ground and out.
Again this is just a suggestion for a family and some of its monsters.

Ok, there. I answered the question.

I don't feel that we need more monster families, as increased complexity with very little depth added is not a good thing.
Just add new monsters to the pre-existing families that have special rules. Like the oiler's and quicksilver's ability to resist and deal elemental damage when burnt/shocked, Trojans ability to do a Shock AoE, Vanaduke's custom defenses, Alpha Wolvers ability to buff nearby monster's attack.

Having monsters be able to do multiple damage types would be okay, as long as they were a super-class to a pre-existing family and thus didn't have all wacky defenses: example, a Drake being a super-class monster to the beast family, being weak to piercing and resistant to elemental like beasts are, but able to do both elemental and piercing damage (breath and claw, respectively)

Having monsters with multiple resistances is okay as well as long as, once again, they were a super-class to a pre-existing family and they only had one damage type that matched the family they were superior to. (or in the case of the oilers and quicksilvers, the visual cues are already well known by the player that fire and shock are generally tied to elemental traps)

Do you want me to add that to my post?

You forgot about "status effects."

Elemental damage is caused by energy-based attacks.

yes, to put it simply but since every type of attack is energy based

Give an example of an energy-based attack dealing non-elemental damage, and I'll gladly attempt to explain it.

Shadow is generally assumed to be Dark Energy in this game. It's a real phenomena, it just doesn't manifest nor is as employable as a weapon as easily as it is in the game. It's not "negative energy", but rather energy from a different source and operates differently.

I think it'd be better if it combined the strengths of both monsters. For example, it takes slightly more damage from elemental, normal damage from shadow, and slightly less damage from piercing, while having the ability to attack with both shadow and piercing attacks.

Pierce is a result of impact damage directed in a certain way that causes to sections of solid matter to be forced apart, allowing passage for the projectile. (physically imparted force, measured in newtons) Now you can argue that "everything is caused by energy" but if the matter didn't constitute a large amount of the attack and wasn't formed in that specific macroscopic shape, it wouldn't be a "physical attack" anymore now would it? It'd be an energy-based attack (see below)

Energy-based attacks operate by overloading or intefering with the targets systems via a dangerous current, (shifting the flow of electrons, measured in joules) causing a chemical reaction on the surface of the target to consume skin, metal, or whatever the target happens to be made of, (alteration of matter, measured in moles, or even joules in a thermal reaction) or by causing deadly amounts of radiation (measured in sieverts)

Shadow-based attacks work by eating away at matter. (the dark energy essentially overcomes the bonding force between atoms, then flings them away from the main body) So it's a physical tearing, but at a microscopic scale.

yes it is and i understand that but the way it's defined in this game is that of "evil" for the lack of a better term

I just gave you a logical explanation of what it was an you outright ignored it.
If it's negative energy then howcome it doesn't cancel out, annihilating into gamma rays and eliminate all life in the nearby vicinity?

I realize that "shadow" is trying to be an "evil" damage type, but as a scientifically minded fellow, I like to find a logical explanation for everything :P (especially valid because there is a mineral called "Dark Matter" in the game, and even more so because Dark Matter attracts Fiends and Undead)
Sorry if I came off prudent, or aggressive, or any other word for it.

I'm active everywhere. I just don't often find threads worth posting in because they're usually discussing things that have already been discussed multiple times along this forum's life.

Anyway, this thread is relevant to a thread I created a while back, and that is new monsters. (especially monsters with additional mechanics)

@ Thinslayer
The characteristics you gave for the defenses/damage types of the Wyrm family are identical to the slime family, so there would be no justification to making them a new monster family.

@ OP
It's always bothered me that there were only 6 families when there were so many more combinations, but never enough to actually make a suggestion about it. This thread piqued my interest however, so I guess I'll say something. As has been said earlier in the thread, the reason OOO hasn't added all the possibilities is probably to keep the game as simple as possible. However, now that the game has been released for quite a while and we are seriously lacking in new content, new monster families would be reasonable with discretion. Wouldn't want all 12 new types to roll out at once, maybe 1 or 2 every few months to a year to give players time to get used to them. New monster families could also generate more gear in the form of monster specialist/split defense armors and weapons that are more suitable to deal with the new creeps.

Responding to your post about monster originality, the almirian legion seems pretty unique, since they seem to have the most lore (displayed in game) behind their creation than any of the other monster types. Constructs are a bit unique for the opposite reason, with some of their original creators and purpose being a mystery. Apparently not all of them were created by the gremlins and could have another purpose other than killing any Spiral Knight they come across in the clockworks. Chromalisks from the beast family are unique, taking the camouflage abilities of a chameleon to sem-invisible extremes and being able to exert enough force to deal pierce with only their tongues (I personally would have thought it only normal damage, but maybe others don't think that way).
If new monster families were ever introduced, with a full roster of 18 there would have to be some larger more defined way of classifying them. Groups that each monster family would fall under, and these larger groups would be the ones that get the UVs and such, so that there wouldn't be an over abundance of combinations when half of them do almost the same thing.

For example:
Resurrected: Kat Klan (formally in the undead family but now gets their own), Owlite Ghosts, Undead
Underworld: Fiends
Wild Animals: Beasts, Insects, Avions
Machines: Constructs
Humanoid: Gremlins, Almirian Legion (formally in the undead family but now gets their own)
Mythical Creatures: Dragons, Fairies, Nature Spirits

These examples were ones I just thought up on the fly, so I'm not too serious about them (I know slimes are missing, couldn't think of a category for them to go with the others). My point being that even if we have more monster families with all possible damage type combinations, keep the maximum of 6 overarching categories to place them all in with 3 families each to keep things simple.