Hi guys! 
I play a yuan ti wizard in the necromancy school. I’m level 5 at the moment.
The other day the DM asked me if I had any ideas for a homebrew signature spell.
Do you guys have any ideas?  The other players in my party are a bard, a rouge, a paladin and a fighter.
At the moment I use my spells to get around problems and let the others do the fighting. For example I used my familiar as a scout when we are in a dungeons or suggestion  against pirates, I had them walk the plank. The other time I used my skeletons to be a bridge over a lava flow and blink to get thrue the DMs jumping puzzle. So think a infiltrator or spy kind of wizard, of course whit aspirations of world domination or godhood. What ever comes firsts.  

Assuming your signature spell has to be form the Necromancy school as you are a Necromancer.....

1) Spectral Hand: Level 2 spell Casting time: 1 bonus action, Duration 1 minute requires concentration (sort of a cross between Mage Hand and spiritual weapon) you summons a spectral hand that can attack. When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon. On a hit, the target takes force damage equal to Id8 + your spellcasting ability modifier. As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it, you can also use it as a Mage Hand. At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by Id8 for every two slot levels above the 2nd.

2) Detonate Dead: Cantrip: Casting time: 1 action, Duration: Instantaneous, You can use your Bonus Action to cause one Zombies or Skeletons you have animated with the animate dead spell to explode in to shards of flesh and bone, the Zombie/Skeleton must be within 60ft of you. Anyone within 10ft of an exploding  Zombie or Skeleton must make a Dexterity Saving Throw, suffering 1d10 Piercing damage on a failed save and no damage on a successful save. This spell's damage increases by 1d10 when you reach 5th level (2d10), 11th level (3d10), and 17th level (4d10).

(As you are playing a Yaun-ti you could switch this to poison damage)

3) Entropic Reaper: Cantrip  Casting time: 1 bonus action, Duration: 1 minute (no concentration required), Components: VSM (requires a staff), You utter an incantation and a curved black blade of necrotic energy forms at the end of your Staff making it resemble a scythe. You are proficient with this weapon, it has the two handed property. If you hit, you deal 2d4 Necrotic Damage but for the duration, you can use your spellcasting ability instead of Strength for the attack and damage rolls of melee attacks using this weapon, If you lose your grasp on this weapon the spell ends.

 Otherwise you can reflavor existing spells for a more necrormantic theme, Clairvoyance re name to eyes of the ghost, functions the same way just  means you are using a ghostly eye instead of a scrying orb, Invisibility/Greater invisibility become Poltergeist Form etc

EDIT:

Spell: Avasculate: level 1, casting time 1 action, requires a ruby worth 50gpas a material component, works like Chromatic Orb but deals Necrotic damage and has the cosmetic appearance of shooting a small beam of ruby red energy at the target, if it hits deal necrotic damage as blood is pulled out of the target and flows back down the beam into the ruby.

not my idea, but my dads, not really a spell either (the detonate dead thing could be used here):

find and reanimate as many rats as possible, if you cant find some, idk make some, or just use any tiny creature, then, swarm an enemy with them, and have them all explode. 

✨NECROSIS RAT NUKE✨