Inspired by the witch from other TTRPGs. 
Witches derive their power from their understanding of the magical patterns of the world and speaking with creatures from other planes to expand their knowledge. Not all extraplanar entities are strong enough to offer a pact. Many are not string enough to resist being bound to a creature as a familiar. It is those in between that the witch seeks out, be they fiend, fey, celestial, or something else entirely.

Familiar Bond

You learn the Find Familiar spell if you do not already know it. You gain the ability to cast the find familiar as a ritual with no material components required, even if you do not have your spellbook.

While your familiar is on the same plane as you, you may cast Hex once at first level without expending a spell slot. This ability recharges on a short or long rest. You may cast it at higher levels by expending higher level spell slots as normal.

You gain the ability to cast Bestow Curse with this ability at 5th level and the ability to cast Eyebite at 11th. 

Potions Mastery

You gain proficiency with the poisoners kit, herbalism kit, and alchemists supplies.
You also gain advantage on medicine checks regarding poison and intelligence checks to identify the effects of any unknown potion or liquid.

Empowered Curses

Your curses are debilitating and your hexes overpowering. 

When casting Hex, Choose two abilities instead of one to give disadvantage on checks related to those abilities, or  you may deal an additional 1d6 of necrotic damage

When casting Bestow Curse, you may choose two of the effects listed instead of one. This does not restrict you from creating new curse effects at the DM’s discretion and using two of those instead

When casting Eyebite, the target also takes an additional 1d6 of necrotic damage from all attacks. 

Familiar Focus

Beginning at 10th level, your familiar learns to take more active steps to protect you from harm and deal damage to your foes.
While within 5ft of your familiar, you may cast spells without a material component as though you were using a spell focus. You also gain a +1 to your AC and Saving Throws. Further, if your familiar takes no action, your spell attack bonus and your spell save DC both increase by 1. 

Secrets of the Coven

At 14th level, you uncover the secrets of the hag’s coven through communication with fey spirits and learn how to use that power to your advantage. 
While concentrating on a spell, you may use an action to choose two other creatures that you can see within 30ft and allow them to use an action to channel the effects of your concentration spell for the duration of the spell, until you lose concentration, or until you end it. Your familiar counts as a creature for the purposes of this feature. 

If both of the other creatures you choose are spellcasters, they can expend a spell slot to cause the effects of the spell to be increased. The power of the spell slot spent by the casters increases the power of the concentration spell by 1 spell slot for every combined 2 levels of spell slots spent, to a maximum of 1+the Witch’s highest spell slot. If both casters spend a level 1 slot on a 5th level spell, combined 2 levels, it becomes a 6th level spell. likewise if both spend a level 2 slot, combined level of 4, the spell increases to level 7.

If one of the creatures uses their action for anything other than using this spell, they will no longer benefit from the spell and any effect that they caused will end. As long as the duration of the spell continues, they may use their action again to use the coven spell.   

If any one of the casters becomes unconscious or incapacitated or otherwise unable to take actions, the the spell no longer functions at the higher level and reverts back to the original level which it was cast. 

For example
If you are using the spell comprehend languages, by choosing two other creatures, you can allow them to use their actions to receive the benefits of comprehend languages until the spell ends or until you lose concentration and end it.
For a spell such as bestow curse, each of the creatures may use the spell as an action to curse a target, even inflicting multiple effects on the same target. Spellcasters May both spend a level one spell slot to increase the spell level and extend the duration as described in the spell description. 
For spells with an area of effect, such as black tentacles, you can cast the spell in an area that does not overlap with the original spell’s area of effect.

After using this feature, you may not use it again until you finish a long rest.