The general hope of this idea is to encourage players to complete the core levels together and smell the roses while doing so.
Add 1-3 (random quantity generation) clear/white-ish/glass mineral spires near loot caches. These always drop 5 medium minerals (one of every color and never any other size) when mined and can be completely mined in one shield bash hit or 5 hits of something else. I like to think that different properties of the mineral "prism" manifest when exposed to air and the energy of a shield bash, like a weird chemical reaction, but since they're in the core they are pure and not stuck in any one color like they are outside the core. Like stem cells I guess. This shield bash quirk is just for fun - it doesn't really matter. Pick one mineral (as always) to carry to the end. Bring friends to get more per level. Mining these minerals sounds like hitting glass, it goes "chink chink chink chink chink (one per color)" really fast and visibly shards off/shrinks when struck by the bash (similar to the noise you get when hitting invincible enemies but higher pitched). If you mine it without the bash, each hit will cause one of the possible minerals to be spit out, so you can stop once the color you want comes out I guess.
New Trinkets:
We'd like to think lost things can be found again. Perhaps they are not as far gone as they seem.
These rings allow you to channel righteous janitor powers through your fingers into your weapons. There are a few types of purifier trinkets that grant different effects and influence combat with The Swarm. Each ring grants a 50% chance for the below. Wear two of the same for 100% get! Or combine them for chances at different awesome cleaning skills at the same time.
Balance: reduce % chance to be usable for a solo player - maybe 12.5%? - but have effects stack for team to encourage large squads of level clearing, so four players each with two rings increases it to a perfect 100% if they're all the same ring. As the new levels are now, people are more interested in speeding through the levels instead of fighting because the monsters don't drop loot, so a lot of the new content is missed or ignored in a rush to get deep for the treasure boxes, which makes me sad. I mean I get why the monsters don't drop loot lore-wise, but as a game to play (vs to read about), it's nooooot the best way to do things. I tried to keep the following trinkets somewhat in-line with the general "swarm is nothing but greed and corruption" idea.
Acquisition: Probably just make these a rare drop from loot boxes in the core.
The rings are pretty function-exclusive, which is something I like to avoid in general. Maybe they could harmonize with Cradle and its clockworks, each moderately increasing the chance to mine fat minerals from single-color mineral spires outside of the core.
1) Vindicator Ring
50% to "1HKO" a swarm entity with charge - this turns it into a Void Soul, and it will attack you as such, but it's much more powerful than the wandering void souls that bother you normally - often 1HKOing you too. It's both vanquished and enraged by all the tuned energy in your charge attack. Hit and dodge 'em, knights! Dashing won't make them vanish like other souls - they're more like bombies than souls really. But still look like souls for the idea. Clear areas faster with a lot more risk - this might encourage players to get into the core more in the future, but doesn't exactly solve the rushing problem on its own.
EDIT: the swarm entity needs to be very tough. Emphasis on "not being a normal soul." Solutions include transformation occurring at the end of a charge sequence only, invulnerability frames, or total invulnerability until the soul executes its explosion.
2) Purifier Ring
50% chance to purify a swarm entity (causes monster to return to whatever it was before getting corrupted, it will then turn into an ally soul and wander off-screen) if you manage to kill the swarmling with a charge attack. As it turns into a soul, it has a 5% chance to drop 100cr (two gold coins). Good job! Get paid with good feels AND hard cash! This is entirely different than the vindicator ring - you gotta work to deplete its hp, not rely on chance to do it for you.
EDIT: for clarity, the transformation is only an aesthetic. The point of the purifier ring is to make you feel good for cleaning the swarm - both your conscience and your wallet should feel good.
3) Embolizer Ring
50% for enemy to drop a single heart if defeated with charge. Gross. Has no effect on normal heart chances, kitty battle sprite's stuff, or other factors.
EDIT: this is a nerfed version of sera's HA. Sera's HA gives defense orbs or explosions in ultimate form. This would only drop single (1 pip) hearts from enemies that rarely drop hearts, and nothing else. This should be used in junction with other trinkets to increase the survivability of a squad with agreed-upon goals.
5) Weeder Ring
50% chance to disable a swarm turret for 7-10 seconds (indicated by the turret turning completely black for the duration) if it's struck with a charge attack. Bombs away! Too bad weeds always grow back if you can't get the roots, and we can't seem to get to the bottom of these persistent turrets.
EDIT: solution for keeping this mechanic from being too powerful: one charge knocks the turret out for X seconds. Hitting the turret again will enable it to function normally.
EDIT: instead of trinkets (rings), call these sprite abilities. We could use more ideas on this front.
If the Vindicator and Purifier rings are used together, and the 1hko chance registers, the pure soul will always be produced but not drop coins because the KO-tuned charge energy brings out intense greed but also purifies (freeing the poor thing from said greed) a moment later. Other effects are in sync and can stack on top of each other. I probably contradicted myself somewhere in here - feedback and thoughts please!
A thought: what if it turns out the swarm only shows us what it thinks we want to see...and the "void" things we fight aren't actually physically corrupted monsters, just visceral embodiments of greed that take on a familiar form, like a mental cookie-cutter, trying to communicate with us in a manipulative way. I often wonder why we don't see greavers, chromas, etc. If this is the case, these trinkets don't make a ton of sense.
I feel like the last thing we need is to remove the difficulty and pure horror from the overarching, ultimate antagonist of the game.
There's no "50% less damage from shadowfire" trinket, or "Extinguish Vanaduke's orbs with attacks" trinket. I think more creative trinkets something along the lines of this would be nice, but not as a belt sander for the sharpest content in the game.