One more thing to think about for all of these - if a player gets a weapon or spell that does any given damage type, what are the "bonus" side effects (debuffs, etc) applied to the enemy? Otherwise, it's just lots of random vulnerabilities / invulnerabilities, aka a giant rock-paper-scissors matrix. Which is fun for certain things, but is not that great in a real-time game with a small, limited inventory. It's more conducive to a game that is turn based with a large elemental "inventory" (whether that's an inventory of spells, weapons, or pokemon), where you have time to find the right tool for each situation. My thinking for Delvers is that the elemental system's RPS nature will only help to vary up a few dungeons, but should not lock players into only fire weapons on level 3 (or whatever). Rather, players might know they need one fire weapon in level 3, but the rest of the weapons they can pick based on what special debuff effects they enjoy using on enemies.
So let's look at all of the elements for their side effects, to see if there is enough variety and differences. One item to note - all elements could do Damage over Time (DoT), though some might be more potent or more likely to do it (so these I will call out specifically).
Simply powerful, especially against the "darker" arcane elements. Side effects would be a high degree of stun and the ability to penetrate through multiple enemies / objects at once.
Summary: 100% Stun
Tangles and slows enemies, and prevents them from using their attacks by binding them w/ constrictive vines.
Summary: Prevents Action, Slows Movement
Can push enemies around, throw them across the room, etc, and drench them, slowing them and washing away any bonus effects they natively have (like putting out the fire on a flaming bat)
Summary: High knockback, slow recovery, negates enemy buffs
Freezes enemies and coats them with ice, slowing acceleration and making them slide around. May also make them slower to react and attack, so all strikes are half the usual speed.
Summary: Slow action, lower acceleration, and lower friction/traction
Pummels and surrounds enemies with cementing stone, weighing them down and preventing movement. Locks them to one spot, like lead or cement boots.
Summary: Complete stasis, slows or prevents rotation (direction changes), but allows action/attack
Rupture / Blast
Very high knockback, has the chance to shatter shield-armor, and may blind and/or stun, reducing enemy pathfinding.
Summary: High knockback, criticals versus armor, blindness
Causes the most damage over time, and enrages the enemy, causing them to attack any objects/entities indiscriminately with a higher than normal speed, causing more fires.
Summary: DoT, Berserker rage, but higher speed
Probably has a high degree of knockback, and causes the shocked entity to cause carry-over touch damage to anything they bump into. May also cause a high chance to misfire or backfire when attacking, causing damage to oneself. Or innervates enemies, preventing them from acting / stunning them? There are already several elements that prevent action.
Summary: Conductive touch-damage to others, much higher chance to misfire / backfire when attacking, but adds lightning damage to all attacks
Blight / Spoil
Poison would cause DoT, and might also be a better placement for the "backfiring actions" than Sunder - so if you are poisoned, any movement or attacks may drain your health much faster. And/or your attacks are simply weaker and do less damage. Could also just make you more vulnerable to physical damage.
Summary: Best bet is probably: DoT, Physical Damage received is amplified, all attack actions do less damage, and some attacks backfire.
Murk / Void
Very high damage, very high chance of critical, and maybe has a chance to cause "doom" with a countdown after which time the enemy instantly dies? Maybe completely bypasses armor (unless it's Splendor-based) and hits Heart HP. This also seems to be a good place to do health-leeching, stealing some of the opponent's health and giving it to you (fits with the assimilation idea of Murk). Vampirism!
Summary: Ignores shields, high chance of crit, may cause instant death or a death countdown, and/or steals health.
Traumatic / Psychic Dread damage. Crazes enemies with fear, causing them to run from the player and maybe even run into pits or hazards intentionally.
Summary: Cray-cray fear effects
Quell / Storm
High knockback, blows enemies across the room kind of like surge. But I also like the idea of a whirlwind that dizzies enemies, causing their ability to rotate to get super wonky, so they can't walk in a straight line.
Summary: Knockback + dizziness
Wild / Roar
This could be a better location for the Berzerker effects (rather than fire). Or if we think of this as the element of savagery and animal life-cycles, maybe it can "eat" the health of the target and add it to the attacker. Or maybe it bloodies and weakens the target, and attracts other enemies to them like sharks to blood in the water. Or... it could put you in a position of dominance to the target, making them your thrall for a time. I like the last idea best I think, so maybe some of the ideas here can go to other elements.
Summary: May cause the damage recipient to become a thrall and fight for you, possibly in a berserker rage (?)
Does little damage on its own, but causes the entity to become confused, use abilities at random, attacking thin air, making its pathfinding really really dumb, so it runs into things and is pretty much a bumbling idiot.
Summary: Confusion, Random action
Silence / Hush
Prevents enemies from using any magical / elemental damage? And this may be the only way to stun or do high damage to spectral / Echo enemies. Could this also be an element of magnetism/gravity/polarity, pulling other objects toward the affected entity? Maybe also prevents healing or something interesting like that?
Summary: Magical Silence, plus...?
Ooze / Plasm / Purge / Bile
One thing that I would attach (if we do this one) is something very similar to Slag in Borderlands 2 - It does little damage on its own, but if you hit it with any destructive elements, they do way more damage. So basically volatile ooze or petroleum tar which can catch fire easily, and is also susceptible to explosive, electric, acid, etc. damage types. ALTERNATIVELY - if we drop this element, its damage amplification could switch to Muddle, Silence, or Blight. May also slow top speed / increases stickiness (traction).
Summary: Amplifies elemental damage but lowers physical damage, and...?
Would also be cool if one of the elements (maybe also Ooze/whatever) dramatically increased the entity's elasticity, causing it to bounce off of everything to the point that it caused damage just by high-speed locomotion. This might replace the stickiness idea of Ooze above, would maybe make more sense if we called the element Plasm?
Any other effects I'm missing? A lot of the other ones I'm thinking of (in other RPGs) would work better in a turn-based game versus a realtime game.