Suggesting some new enemy designs (can also combine some ideas):
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Foe that teleports closer to you when it is hit by a projectile
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Foe that teleports away from you when it is hit by a melee attack
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Foe that strafes sideways the moment a projectile is thrown/shot in their direction
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Foe that slowly moves away from you if you get close, but leaves a trail of acid behind as they do
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Foe that hangs from the ceiling. If you shoot them down, they land and crawl towards you
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Foe that creates smokescreens or fog to hamper visibility
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Foe that creates illusionary duplicates that mimic it and you need to hit the correct one
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A slow, but exceedingly large foe that takes up 90% of a hallway in width
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A small foe that jumps up and down in place and fires projectiles from different heights during its jumps
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A transparent foe that becomes more opaque the closer they are to you
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An invulnerable foe with weak points you need to aim for and destroy
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An invulnerable foe that doesn't turn very well and needs to be circled around and hit from behind
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An invulnerable foe that throws projectiles you must hit to reflect back at them
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Foe that leaves a puddle of goo when killed, which slows you down
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Foe that leaves a puddle of acid when killed, which damages you
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Foe that leaves a puddle of oil when killed, which can ignite to create a fire hazard
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Foe that leaves a patch of ice when killed, which is slippery and makes you slide on it
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Foe with an aura that slows down your movement
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Foe with an aura that replaces all audio with a static sound as if deafened
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Foe with an aura that scrambles the view of the mini-map & health remaining
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Foe that charges up a firebreathing attack, and can't move or turn while breathing fire
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Foe that slowly pushes you away (for example, by flapping wings to produce wind)
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Foe that slowly pulls you towards it (for example, a floating orb above a trap or a pit)
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Foe that pulls you towards them with a chain/whip/rope/web/vines/etc.