I am making a Witcher-style monster hunting campaign for my friends, where monsters have specific weaknesses and such. I would love for suggestions on monsters I could use (Monster Manual, preferably) and different weaknesses I could add in...

I will say, I have modified several monsters already (Zombies with Pack Tactics and Keen Hearing and Smell >:D) and edited different vulnerabilities. This is my first campaign, and I would like any help I can get. thanks!

Some suggestions:
Bugbear
Cyclops (Poor depth perception)
Doppelganger - have it fake being one of the party or an NPC - lots of fun

Do you have ay PCs that are Blood-hunters?  Seems to me this is a perfect Witcher-style class.

Okay, so of course I can't think of any of them off the top of my head, but there are some undead that are vulnerable to radiant damage (or you could make them vulnerable to it even if they normally aren't). You can always just pick a monster and make it vulnerable or resistant to a damage type that makes specific sense to you even if that isn't in the stat block. (i.e. making a stone golem vulnerable to cold damage and frost attacks in particular because freezing water can crack stone.)

Essentially, don't be afraid to homebrew to fit what you need.

hmn, sounds interesting, the ice thing...

I am currently asking my players if they are okay with the Blood Hunter class, as some of them find it slightly disturbing. I was considering Gnolls, but I didn't think of Bugbears!

I am also making several monsters vulnerable to Silvered Weapons, to make them essentially a requirement and mess around with my players when they do crappy damage against foes >:). I will also be using the Witcher's method of Poisoning yourself to stave off bloodsuckers, but am instead using Holy Water instead, and Blessed blades.

Sounds like you've got this, you're good.
Hope you and your players have a great campaign.

really? Thank you!

Quote from A_DnD_Noob >>

hmn, sounds interesting, the ice thing...

I am currently asking my players if they are okay with the Blood Hunter class, as some of them find it slightly disturbing. I was considering Gnolls, but I didn't think of Bugbears!

I am also making several monsters vulnerable to Silvered Weapons, to make them essentially a requirement and mess around with my players when they do crappy damage against foes >:). I will also be using the Witcher's method of Poisoning yourself to stave off bloodsuckers, but am instead using Holy Water instead, and Blessed blades.

 Look at werecreatures. Definitely silvered weapons help there...

I have made a list of Creatures, regions they will appear in, and weaknesses- I will send it later for advice and opinions!

(The list may seem all over the place, but I really am focusing on a fight-or-flight style system, where you could only encounter Gnolls or run into a Dragon first try)