I am also playing a nethroi deck.
Fertilid - im not sure if you have any mana issues. but
fertilid is a great addition, and ill come back as a 2/2 every time you mutate nethroi.
Fiend artisan +
lurrus of the dream-den +
alseid of Life's bounty - this combo here will give you unlimited creature tutors. grab your
fiend artisan, bring out lurrusof the dream-den, next bring out
alseid of life's bounty. use
alseid of life's bounty as your sacrifice target and just bring him back for 1 mana every turn. OR! if you dont need to tutor out a creature, keep him around (or bring him back) as protection for one of your mutate stacks!
Fiend artisan could supplement or even replace
Razaketh, the Foulblooded. Razaketh searches for "a card" which is great, but he hurts you and costs a lot. instead..
Sidisi, Undead Vizier +
Boneyard Lurker - These two cards are great together. tutor out sidisi. target sidisi as the "exploit" creature and get your card. then mutate
boneyard lurker to get sidisi back to your hand (or mutate nethroi to get sidisi back. every time Sidisi enters the battlefield you can exploit him for a card)
Gemrazer - 4 cost for a 4/4 with reach and trample. plus whenever you mutate him you can kill off an artifact or enchantment! you need this card in there.
Vulpikeet - a 3 cost mutate that gives you +1/+1 counters on mutate, and flying. i love starting a mutate stack with this guy.
Kaheera, the orphanguard - 3 mana for a 3/2 with
vigilance, plus he gives all of your baddies (most of your cards are going to be affected by him)
vigilance and +1/+1.
Sawtusk demolisher - 6 mana (or mutate for 4 mana) for a 6/6 with trample and removal. id rather give an opponent a 3/3 token than deal with a lot of enchantments (or kill their land if youre a jerk)
Necropanther - bring back your dead! i normally use him to bring back
lurrus of the dream-den if he gets killed off early.
shadowspear - give a big dude
lifelink and tample and +1/+1, also will let you take out pesky indestructible creatures
The ozolith - keep all those counters and use them again! i love this card. pair it with a
hydra's growth.
Silent arbiter - this card can make a game just.. not fun. but it can win the game for you if played at the right time. its very easy with mutate to get your commander to be essentially unblockable. if you just mutate nethroi ontop of something with menace.. he will turn into a problem real fast.
My favorite is putting
Polukranos, unchained +
Cavern Whisperer +
nethroi, apex of death together. this will make your nethroi an 11/11 and unblockable (if you have
silent arbiter out). even without
silent arbiter, it makes nethroi essentially immune to deathtouch for a few turns at least.
Mirror shield would also give you more hexproof for your mutates as well, and make it immune to deathtouch.