Right now there are 3 turrets in the game
Puppies (Construct)- Deal Elemental, weak to elemental
Howlitzers (Undead)- Deal Shadow, weak to elemental
Polyps (Slime)- Deal Pierce, weak to shadow

okay see the problem here Jenkins?
none of them are weak to piercing damage, making howlitzers fiends could potentially fix this, but that's boring
so, incoming turret ideas!

Chaos Crux (fiend)
Tier 1 - small floating purple orb with a red eye, shoots a small laser of dark energy
Tier 2 - Slightly larger, pulsating red eyes, shoots 2 small lasers of dark energy but in opposite directions (eg: one goes north, other goes south)
Tier 3 - Same size, has dark spikes coming off it, shoots 4 small lasers but in 4 directions
Comes in Electric, Fire, Ice and Poison Variants, Tier 3 and shadow lairs can spawn a curse variant on rarer occasions
we saw something similar in the fiend rooms, except these don't constantly spawn fiends

Dingo Drakes/drakolisks (beast)
Tier 1 - a small dragon whelp about the size of a silkwing, similar to a drakon or chromalisk, shoots a spike
Tier 2 - bigger Dragon whelp about the size of a tier 1 slime, same design as drakon or chromalisk, shoots a faster spike and can buff nearby beast types with a defense buff
Tier 3 - a dragon whelp about the size of a tier 2 gremlin, same design as other tiers but more scaly, shoots a spike and can buff nearby beast types with a defense buff, capable of slow movement.
Colours will be similar to drake scail mail for normal, then comes in poison fire etc. no curse. we have dragon mails but no drakes to slay, drake scales can currently drop from chromalisks, but those are more lizard than dragon so... DRAYGUNS!

I would add a gremlin but lets face it, it would probably just be a mortafire
add ideas as you like