So I have a lot of variant ideas which have big behaviors:

King/Megaboss: Has 32x HP, Has 16x DMG, it also can spawn a random variant npc/snpc from an addon.

Illusioner: Disguises itself with a random variant enemy that is currently on the map, this makes the GUI think it's that enemy and it's variant. once it is at 25% hp, it will reveal itself what it really is and the GUI will show it properly. (to identify what it is, look for off setting behavior.)

Miniboss/Elite: 4x HP, 2x DMG.

Mech (Boss Variant): Because there is such thing as Boss Variants now, why not suggest one? This variant spawns with 5000 HP and 50% armour, It has a killable turret on it's head with 1250 HP, They have a 5% chance to throw a droid variant, a small version of the mech which have 500 HP and 25% armour. when at 50% HP, it will go in rage mode, getting 35% faster and having 21% more DMG. When killed, it will do 6 big explosions and then die.
(Spawn message: Here he comes!)
(Spawn method: Random spawns or 1000 enemy kills)
(Near spawn message: You hear rumbling in the distance…)
(Boss Type: Tank)

Leaper/Frog: Leaps instead of walking. It's leaps get bigger the higher the level.

Puzzle (Miniboss Variant): Why not suggest minibosses too? This variant has 5 HP, It is immune to every projectile shot at it, when it attacks you, you will be shown a puzzle, it will make 4 clones of itself that are red, blue, yellow and green, they will make a sound in a order, shoot the clones in that order to deal 1 damage to Puzzle. If you get the order wrong, something bad happens, like spawning a enemy near you or healing itself by 1 HP.
(Spawn Message: A Puzzle is waiting to be solved!)
(Spawn method: Spawns randomly)
(Miniboss Type: Basic)

Guardian (Miniboss Variant): This Miniboss Variant is a toughie, Having 500 HP and 4 metal shields with 250 HP around it, It can use powers to throw nearby props at you, It even has a damage limit of 10 meaning the max damage you can do to it is up to 10. After 30 seconds it can also regenerate it's shields if they are broken. When it's at 50% HP, it will start to heal itself and enemies and even give force fields to enemies nearby it.
(Spawn message: You have upset a Guardian!)
(Spawn method: When you kill 150 of an enemy you killed.)
(Near spawn message: Looks like something doesn’t like what you are doing…)
(Miniboss Type: Support)

Elemental: Slows you down, burns you, makes you weaker and electrocutes you, sounds like a op one tbh.

Earth: Weakens you to 1.5% of your DMG, Up to 25% DMG at LVL 100. It also has 10% less speed.

Lightning: Electrocutes you, it's like the fire effect but every time it hurts you it stuns you for 2.25 seconds. It's also 10% faster.

Fuser: This variant likes to get in other variants personal space, but put another variant next to it and that other variant next to Fuser and bam! A Mix2 variant! It has 45% of it's HP so kill it fast.

Swinger: This variant can grab props and spin around with the prop and throw it at you. Higher force the bigger the LVL.

Glitch (Miniboss Variant): This Miniboss Variant is very hostile, and can rapidly change variants when shot (Except for the following variants: Miniboss, Boss, Bigboss, Megaboss and Miniboss and Boss Variants.). It has 175 HP and has a DMG limit up to 10 like the Guardian. It regens 3 HP per second and can change it's speed.
(Spawn message: 01111001 01101111 01110101 00100000 01100001 01110010 01100101 00100000 01110011 01100011 01110010 01100101 01110111 01100101 01100100)
(Spawn method: Spawns randomly or 2.5% chance when Fuser fuses.)
(Miniboss Type: Basic)

Latcher: Small vicious fast monsters that like to leap towards you and grab onto you, draining your health. Press the E key rapidly to shake them off, thus killing them.

Crazy/Sugar Rush: Extremely wild variant that has very fast twitching movements and does very fast dashes towards you, do not let this variant get close to you as it attacks extremely fast. (It can explode on death too, so don't use melees on this one.)

Jetpack: This variant loves using it's jetpack. It can fly in mid-air ready to strike down at the player once close enough, they have 75% of their HP but are slightly faster. It can easily be taken down once you shoot it's jetpack, causing it to blow up and die.

Shield: This variant has a wooden shield with 75 HP and 15% DEF. The enemy itself has a slight HP, ATK, and knockback increase by 10%.

Champion (Boss Variant): This variant is very battle-hardened, Having a metal shield with 750 HP and 35% DEF. He also has armor on with 2500 HP and 50% DEF. The enemy itself has 1750 HP and 75% ATK and 25% DEF. When he is at 50% HP, He goes into rage mode, making him do 44% more DMG and knockback. He also can summon other variants too. With a 15% chance, it can stun you for 7.5 seconds.
(Spawn Message: A challenger has entered the ring!)
(Spawn method: Random spawns)
(Boss Type: Tank)

Impostor: When this variation spawns, it will look like a player, and select a random PM from your list. This variation also acts like a player, making it hard to know if it's an enemy. It will randomly try to attack you, when killed, it will reveal itself and die.

Plastic: 7.5% More HP, 2.5% DEF and ATK.

Wood: 15% More HP, 5% DEF and ATK.

Stone: 30% More HP, 10% DEF and ATK.

Metal: 45% More HP, 15% DEF and ATK.

Obsidian: 60% More HP, 20% DEF and ATK.

Bedrock: 75% More HP, 25% DEF and ATK.

Mutant: This is a very weird variation, this variant is different every time it spawns, Random HP, speed, ATK, DEF, and abilities every time it spawns.

Rammer: 67% more HP, This variant is angry and triggered, randomly ramming into you when you are close to it. when it rams into a solid object, it gets stunned for a few seconds.

Flinger: Tosser variant but with weaker throw force.

Thief: This variant is 35% more faster, don't let it get too close or else it will steal your items. When it's killed it will drop the items it stole. (Thief can steal up to 5 items, then it will run away.)

Bird: This variant is like jetpack, but it's smaller, weaker, and faster (35% less health than Jetpack and 60% more speed to be exact). Upon death, it has a 6.5% chance to drop an egg, spawning a Hatchling.

Hatchling: Offspring of the Bird. It can't fly, but it can leap towards you.

Healer: This variant can't attack but any enemies nearby it will gain 5 health per second, including the Healer itself. (Healer can't heal shields)

Repairer: The Healer's mechanic worker friend. This variant can't attack like the Healer but it also can't heal nearby enemies, but it can repair/heal enemy shields by 5 health per second also healing the Repairer itself.

Present: I was going to suggest this on December but nah, anyways this variant has 17.5% more HP and it carries a present on it's back, when killed, it will drop it's present. When you go near the present and press E on it it will open, giving you weapons and other goodies.

Magnet: This variation has 27% more HP and it can attract nearby props and throw them at you, it can even defend it's allies by putting a prop in-front of them.

Savage: Very mad and enraged variation. It has 2x ATK speed, 3x Moving speed, and 2.35x ATK. It can even leap at you if you are far away from it.

Inverted: This variant goes by it's name, it has inverted colors, but when it hits you, your controls get inverted. Also dealing damage to it won't do anything, you have to heal it! (trust me, it damages it.) Try using a healing weapon or putting a variant that heals next to it to kill it.

Poison: I think somebody suggested this but eh. Simple poison type, like the fire effect but does 7 damage per second.

Spliter: This variant is very self-dividing, as when it can split more, it's bigger and has more health. It can split with 1/2 being the weakest and 1/10 being the strongest. They can get stronger when they get closer together, fusing into a greater Spliter and will evolve more when the Spliter being fused with is a higher level, if 2 1/10 Spliters fuse it will form a Mega-Split.

Mega-Split (Boss): The father of the Spliters, Mega-Split has 3225 HP and it can split into 1/25 all at once when killed, it can also throw Spliters that can split into 1/2, the Spliters will split at 1/4 when Mega-Split is at 1/2 of it's HP.
(Spawn Message: Be prepared for spliting madness!)
(Spawn Method: Spawns randomly or when 2 1/10 Spliters fuse.)
(Boss Type: Basic)

Hope these get added!
(Also credit to Vladbzf for the Magnet variation and Savage variation, I just improved them a little bit.)