If we’re just talking core concepts, I think there’s not a lot of point in focusing on specific abilities/themes/names yet. So I’m just gonna focus on very broad concepts.
Rogue: If Goliath and the Kraken are the warrior and the wizard, then it makes sense to have a rogue, a monster who is physical frail but uses speed, high damage, and stealth to counteract that. It’d probably need some ability that allowed it to separate the hunters more effectively.
Broodmother: Kraken might be the wizard, but there’s more than one kind of wizard archetype in gaming, and one of them is the summoner. A monster that can spawn lesser monsters, or control the wildlife or the bodies of dead hunters, etc, might have potential.
Tank: Goliath is a nice balance of toughness and strength, but what about a monster who was more skewed towards toughness? Maybe it doesn’t hit as hard, or maybe it’s a bit slower, but it’s got a ton of health and armor. A monster like this might have to rely on ambushes, hiding in plain sight like a Megamouth, because it’s not as easy for it to just run, run, run.
Trickster: A monster that relies on tricking the hunters’ senses. Without getting too into potential abilities, its main goal would be to have the hunters going in the wrong direction and shooting at the wrong thing, while it runs around and eats. Then, while the hunters are distracted by illusions or false trails, it attacks.
Controller: All of the monsters so far have had one or two abilities that are useful for crowd control, but what if there was a monster that had that as its specialty? It would focus on debuffs and area-control. Its abilities might do less damage, but by controlling the battlefield it could open the hunters up to melee or sneak attacks.
I feel like one big limit to new monster concepts is locomotion. Any terrestrial creature is going to be limited to some form of walking or flying and there only so many ways you can shake that up. Swimming monsters are probably out of the question, given the map design we’ve seen so far. So the potential number of different monsters is probably going to partially be constrained by how many different forms of movement Turtle Rock can come up with. Here are a few ideas they might use:
Wall Crawling: The monster is not substantially faster than normal, but it can climb on vertical surfaces, possibly even on the ceiling of cave like areas. I actually think this one is unlikely, as it would be really hard for the devs to map sure every wall and ceiling could allow wall-crawling. I mean, think of those spiky ceilings in the caves on The Dam.
Burrowing: The monster burrows underground, where it can move faster. It has to un-burrow to cross rivers, chasms, and to climb walls, and it leaves trails of turned earth like a mole that replace its footprints.
Teleportation: This might work for a wizard-esque monster with weird energy powers, similar to Kraken. It’d probably have to be a simple “blink” ability, with the monster teleporting to wherever its reticule is aimed, with a limited range and without the ability teleport through walls. It’d make tracking the monster rather hard, but perhaps a massive burn mark would appear both where it teleported from and at its arrival, so hunters could potentially spot both and make a guess at its path.
Super-Speed: The monster runs normally, but when it uses up one of its stamina bars it can temporarily run at incredible speeds that leave even the other monsters in the dust. The recharge on its stamina would probably have to be slower than usual, otherwise the hunters might never catch it.
Chameleon: When the monster isn’t moving or when it is sneaking, it blends in with its surroundings. This is only sort of a movement ability, but the idea would be that instead of outrunning the hunters the monster would try to just hide from them, then go off in a different direction. Of course, for the sake of balance, it couldn’t be entirely invisible.