Here is a concept for a new "regular" monster mob considering our roster right now is fine, but needs an upgrade to make survival a tad more challenging. Since I am a huge fan of ghosts and spirits, why not something like this? (Not Phantoms since Spirits wouldn't care how much sleep you have).

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Spirit
Health: 60 HP (30 ❤️).
Speed: 10 SP normally, 30 SP when hostile.
Damage: 2 HP (1 ❤️) melee damage, 1-34 (0.5-6.8 ❤️) through tossed blocks, dependent on weight.
Behavior Class: Hostile (at night or when attacked), Neutral (at day).
Drops: 1-2 Ecto-Slime Ball, 2-4 bones, 14 EXP orbs.
Category: Undead, Monster.

Spirits are neutral undead mobs that spawn occasionally at night in the Overworld except the Deep Dark and Igenous Caverns. They are normally invisible to the player and are able to pick up items and move them around. Spirits are capable of moving through any blocks except for bedrock, obsidian, reinforced deepslate, and barrier blocks. They are normally hostile at night and will toss any item(s) they have on them at players until they are attacked, of which they would reveal themselves. In the day time, Spirits would become passive, and can burn in the sunlight if the weather is clear, will try to enter caves & houses that are at least 4 blocks tall, and will only attack players if they are provoked.
Spirits are known to drop 1 to 3 ecto-slimes at death, a unique edible item. 4 Ecto-slime balls can be used to craft an invisibility cloak alongside 4 leather, or used to brew intangibility potions, which grants players the Spirit's ability go through blocks and levitate controllably for 30 seconds (Intangibility I), 2 minutes (Intangibility II), and 10 minutes (Intangibility III). When eaten, Ecto-Slime balls have a 25% chance to give a player the Intangibility Effect for 10 seconds and 75% chance to give nausea II and blindness for around the same time.

Notes:

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1. Spirits are inspired by Poltergeists and some modern depictions of other ghosts.
2. A spirit cannot pick up Ecto-slime blocks, or any blocks they cannot fly through (Bedrock, Obsidian, Reinforced Deeplsate, and Barrier). They also cannot leave the World Border.
3. Spirits count towards the Monsters Hunted Advancement and have two Advancements of their own: Spirit Disclosure, granted when a player first sees a Spirit's true form, and You Can't Catch Me, which is granted when a player eats an Ecto-Slime Ball, and gains the Intangibility effect.
4. Spirits come in many different colors and appear as translucent, vaguely humanoid mobs that are wearing clocks, have glowing white eyes, and are entirely one color. They are 2.36 blocks tall and 1 block wide.