A few Points, split into two parts (Difficulty and Feature Suggestions) (Note i have not played the newest version yet, and cannot give my personal Input on that as of the moment as i've only seen the patch notes, though i do plan to play it later.)
Poltergeist's "Overhead Swing" attack feels as if it has FAR too much range, while their "Spin" attack is a bit too frequent/easily chained together. One Poltergeist on it's own is fairly manageable, but two or more result in a MASSIVE leap in difficulty, especially when you're being hounded by one that's Spinning while the other is doing the Overhead attack. Although i believe this IS significantly less dangerous in the new version (now that parrying doesnt drain Stamina.) This is what i'd rate as the #1 most dangerous enemy in the game.
Miasma's "Siphoning Leech" feels INCREDIBLY strong, doing somewhere around 25-30 damage in combination with the hit that inflicts it. While it's dangerous in the Nethercrypt, it's often because there's another enemy rather than this enemy alone (See: Poltergeist).
Lord Kaluuz (The Nethercrypt's Boss), overall, has a few things i'd like adjusted (this may have been partially fixed in the newest patch). Primarily, how the two Ranged attacks function:
The "Orb" attack, and the "Cross" attack. The former i feel could do with weaker homing (the only method i've discovered to consistently avoid taking large amounts of damage is to dodge to the left and a bit into the attack, NEVER use block.).
The latter is overall the more punishing/brutal attack. As of right now, i have NO idea how to avoid it, and my strategy is only to "get fairly close to the boss and block", to avoid getting knocked back into where the AoE persists, resulting in my Stamina being drained instantly (followed by a large portion of my HP)
Both run a VERY real risk of "Juggling" the player if they ever attempt to block either attack, almost always draining your Stamina if you take a direct hit. Additionally as far as i can tell there's no way to "Parry" the attacks either, or interrupt the boss (without killing it, at least.)
The Melee attack COULD perhaps do with a very slightly longer telegraph, but not really necessary. And the Beam attack is fine as-is. I did find quite a LOT of attempts died either from being Juggled and having my Stamina drained, being hit by the Poltergeist Overhead, or hit with the Miasma's Siphoning Leech.
Finally, the last detail of note would be enemies doing Critical Hits. It's not often, but sometimes it does hurt when a normally survivable attack from a non-boss (eg Slime Diva or Poltergeist) is a Critical Attack that does more damage than your player's HP (eg a Lv 10 Mystic vs Slime Diva (approx 30~ HP vs a 34 Crit), or a Level 20 Mystic vs a Poltergeist (approx 50~ HP vs a 54 Crit)).
Perhaps a small change in which receiving a Critical Attack that does more than your Max HP would instead reduce you to 1 HP? (Of course, ONLY for attacks that would otherwise not kill you in one hit. A 19 HP Player taking a 20 damage hit should kill regardless of whether it was a Critical Attack or not.)
Difficulty aside, now for Features i'd like to see in the future (roughly in order):
1 - Weapon Transmutation/Tweaks to Transmutation: As others have mentioned, Transmutation at the moment only feels like something you can do near Endgame, when you've got tens (or even hundreds) of thousands of Crowns to spend, with Illusion Stones at a whopping 1k per (or 5k Crowns for all 5 slots on a character), especially when swapping or removing that equipment erases the Transmutation, making doing so earlier on completely impractical when you're upgrading so often.
How i would set it up is either: Attach the Transmutation to the equipment piece itself (Guild Wars 2 has a VERY similar system to this, including even a consumable called "Transmutation Charges") OR Attach the Transmutation to the vanity slot itself (although i feel it should still dissipate if no item is in said slot, otherwise i'm unsure how you would be able to remove/hide one such as the Shield Transmutation)
Additionally: The option to Transmute your weapons as well as shields. Say, for example, Geistlord Claws > Runic Katars, Nethercrypt Bauble > Pyre Cane, or Nethercrypt Blade > Dawn Mace (Although i'm unsure how this would affect interactions specifically, if Coldgeist Frostcaller > Wooden Bell would change the attacks of the Coldgeist Frostcaller to that OF the Wooden Bell, or if it would remain the original attacks/element but with the new skin)
2 - Grimoire/Book Weapon Type: This is fairly simple (in concept, at least). Instead of a Scepter or Bell, you could wield a Book. Although i'm unsure how EXACTLY it'd function. Perhaps as a few ideas for how they'd function, it makes spells (eg Fluxbolt) cast faster, or maybe it lays down AoE zones, or acts as a "Beam Weapon"/"Channeled Weapon" of sorts, or even summon minions to fight for you? But certainly Magical Books are neat (and i feel it'd look great with my Mystic in the new Boss Set, still not sure if i like Witchlock or Witchwizard Robes more at the moment), overall very nice looking, potentially very large...
3 - An alternative source to Prefixed items: This one is very much optional, but it would be nice if, later into the game/perhaps after a Quest, you could unlock an NPC or Station that would allow you to manually "Enchant" an item for a high cost (perhaps resulting in a random Prefix as a result?). I've noticed that some items where Prefixes ARE a possibility tend to sometimes so rarely come with them/drop that often it'd take an absurd grind or significant luck to obtain. (eg some of the Geistlord Items, the Menace Bow from Crescent Keep, or Slimek/Deathgel Shivs/Geist Scythe. Although i cant confirm they ARE super rare, this is just personal experience/may just be terrible luck. Deadwood Axes were a similar case until i dropped like 7 Prefixed ones in a short span of time)
4 - Additional Equipment/Weapons: This is already a given considering the game is most CERTAINLY going to get more content as time goes on (as the version i'm writing about proves and emphasizes, being absolutely PACKED with new items that already address this point), but for the moment a handful of weapons/equipment are comparatively lacking. (I'm aware there's a new item in the latest version which may very well be one of these categories as well.)
Primarily they are from least to most: Bell - 3 Items (Wooden, Cryptcall, Coldgeist), Polearm - 4 Items (Wooden, Cryptsinge, Sinner, and Necroroyal), Cape - 5 Items (Initiate, Rugged, Nethercrypt, Regazuul, Deathward), and Shield - 6 Items (Wooden, Crypt/Demicrypt/Nethercrypt, Iris, and Omen).
5 - Pets/Companions: By far the most unnecessary/Optional of my requests/suggestions as of yet, feel free to Disregard. The ability to obtain a Pet (or hire an NPC at the Hub? Unsure if they'd be a Creechur similar to the Player or Guardians, or just something hanging around/unlocked.) and equip them to a slot. Potentially either fighting for you directly, or acting as a Support that buffs (or heals) you during a fight. (If the NPC/"Mercenary" route rather than a summonable minion that'd hover around you, perhaps said NPC even having a separate Health Pool, and automatically reviving after a set cooldown/when you interact with the body? This would be a bit *Too* much on the technical side to ask for, admittedly.)
Overall good game, DEFINITELY cant wait for the new releases/new areas, alongside any new enemies and NPCs. (Such as the new dungeon entrance in the latest teaser in the Discord Server, released right in the middle of writing this.) Perhaps even the "Advanced/Master Classes" hinted at in the Skillbook?
(And perhaps i DID ramble on a bit too long on some things :V)