I don’t like Find Familiar or Conjure X spells in 5e D&D, and I’ve been circling around the idea on how to fix them for a while. I think this is how I will do it next time I run Type V–spells that call up a critter what does as you say, but prevents the caster from recovering the spell slot used until the creature is dismissed. This sort of amortizes the benefit of a spell slot over the course of a day, as opposed to making them a single-use get-out-of-jail-free cards. It also gives me a measure for what these creatures should be able to do–there needs to be a reason to pick this spell over any other 1st level spell. Finally, it means I don’t have to fuss with class mechanics, which is a big plus. How to implement this:

  • Wizards can get this spell as is.
  • Warlocks can get this as an Invocation that lets them use a Warlock slot to cast it 1/day.  (Being able to cast it 1/short rest would let warlocks be able to easily circumvent the cost of losing the spell for the day, since they could just dismiss the creature and take a short rest.)

I’ll probably make a version for 3rd, 5th, 7th, and 9th levels, as well. Stronger versions might only be for a single creature.

Minor Covenant
1st-level conjuration 

Casting Time: 1 hour
Range: 10 feet
Component: V, S, M (an opal, ruby, piece of quartz, or sapphire worth at least 50 gp)
Duration: Permanent

You summon a lesser spirit that appears in an unoccupied space that you can see in range. A spirit summoned by this spell disappears when it drops to 0 hit points or you choose to end the spell, which takes an action. As long as this spell is in effect, you cannot regain use of the spell slot you used to cast it.

The kind of material component you use determines the kind of lesser spirit this spell summons.

  • An opal summons a sylph
  • A ruby summons a least djinn
  • A piece of quartz summons an true gnome
  • A sapphire summons an undine

The spirit will not attempt to harm you or your companions. It receives its own place in initiative. The spirit obeys any verbal commands you issue it (no action required by you). If you don’t issue any commands to it, the spirit will follow you without taking any other action.

As an action, you can temporarily dismiss the lesser spirit. It disappears into the Void where it awaits your summons. As an action while it is temporarily dismissed, you can cause it to reappear in any unoccupied space within 10 feet of you.

The spirit can break free from the bonds of this spell. It may make a Wisdom save whenever

  • you give the spirit a new command
  • you insult the spirit or place it in egregious danger
  • you command the spirit to perform an action that requires it to ignore one of its compulsions or perform one of its taboos.

If the spirit succeeds its Wisdom saving throw, it breaks free from your control, becomes hostile to you and no longer obeys your commands. You cannot temporarily dismiss it, and ending the spell no longer causes the spirit to disappear, though it does allow you to recover the spell slot used to cast this spell.

Sylph

from bravely default

Small elemental, unaligned                                                          

AC: 15
HP: 6
Speed: 0 ft, fly 30 ft.                                              
STR: 4 (-3) DEX: 20 (+5) CON: 8 (-1)
INT: 10 (+0) WIS: 10 (+0) CHA: 12 (+1)                   
Skills: Perception +2, Sleight of Hand +7, Stealth +7
Damage Resistances: slashing, piercing, bludgeoning
Languages: Common, Sylvan
CR: 1/8                                                         

ABILITIES

  • Innate Spellcasting (at will): The sylph can innately cast the cantrip Gust. Its spellcasting ability is Charisma. 
  • Keen Smell: The sylph has advantage on Wisdom (Perception) checks that rely on smell. 
  • Ethereal: Sylphs can squeeze through spaces as if they were Tiny creatures

                                                                              
ACTIONS

  • Superior Invisibility: The sylph magically turns invisible until its concentration ends (as if concentrating on a spell). Any equipment the sylph wears or carries turns invisible with it.

    DISPOSITION
    Sylphs are spirits of air and propriety. They despise rudeness, ugliness, and violence, and delight in courtesy, beauty, quietude, and refined conversation. Despite their ostensibly peaceable and orderly natures, sylphs will steal, murder, and torment to punish those they have deemed transgressors. 

    from final fantasy 12 revenant wings

    True Gnome

    Medium elemental, unaligned                                                    

    AC: 11 (natural armor)
    HP: 19
    Speed: 30 ft                                                              
    STR: 15 (+2) DEX: 10 (+0) CON: 14 (+2)
    INT: 4 (-3) WIS: 12 (+1) CHA: 12 (+1)                   
    Skills: Perception +2, Nature +4
    Damage Resistances: slashing, piercing
    Languages: Common, Celestial                                 
    ABILITIES

    • Innate Spellcasting (at will): The true gnome can innately cast the cantrip Mold Earth. Its spellcasting ability is Charisma. 
    • Quiet Strength: The true gnome has advantage on Strength checks to lift, carry, bend, break, and otherwise manipulate inanimate objects

                                                                                  
    ACTIONS

    • Punch: Melee Weapon Attack. +4 to hit, reach 5 ft, one target. 2d4+2 bludgeoning damage.

    DISPOSITION
    Ursulines are spirits of earth and obligation. They dislike dishonesty, destruction, and disorder, and appreciate respect for wildlife, honesty, prudence, and sleeping. 

    from final fantasy

    Least Djinni

    Small elemental, unaligned                                                    

    AC: 15 (natural armor)
    HP: 2
    Speed: 0 ft, fly 40 ft                                                              
    STR: 18 (+4) DEX: 12 (+1) CON: 10 (+0)
    INT: 10 (+0) WIS: 8 (-1) CHA: 14 (+2)                   
    Skills: Stealth +3, Athletics +6
    Damage Resistances: slashing, piercing, bludgeoning
    Damage Immunities: fire
    Languages: Common, Primordial                                 
    ABILITIES

    • Innate Spellcasting (at will): The ursuline can innately cast the cantrip Control Flames. Its spellcasting ability is Charisma. 

                                                                                  
    ACTIONS

    • Discern Desire: The least djinni touches a creature and magically knows what the creature most desires at that moment. 
    • Bite: Melee Weapon Attack, +6 to hit, 1 reach 5 ft, one target. d4+4 piercing damage, and the target must succeed in a DC 10 Constitution saving throw or become poisoned for 1 minute.
    • Invisibility: The least djinni magically turns invisible until it attacks or casts a spell, or until concentration ends (as if concentrating on a spell). Any equipment the least djinn wears or carries is invisible with it.

    DISPOSITION
    Djinn are spirits of flame and desire. They dislike equivocation, moderation, and cowardice. They love the exertion of power, achieving desires, and eating.

    Undine

    Medium elemental, unaligned                                                    

    AC: 2
    HP: 9
    Speed: 25 ft, swim 40 ft                            
    STR: 6 (+2) DEX: 12 (+1) CON: 8 (+2)
    INT: 11 (+0) WIS: 10 (+0) CHA: 16 (+3)                   
    Skills: Persuasion +5, Performance +5
    Damage Resistances: slashing, piercing, bludgeoning
    Languages: Common, Primordial                                 
    ABILITIES

    • Innate Spellcasting (at will): The undine can innately cast the cantrip Shape Water. Its spellcasting ability is Charisma. 
    • Translucency: The undine can become invisible in water as a bonus action
    • Amorphous:The undine can squeeze through a space as narrow as 1 inch without squeezing

                                                                                  
    ABILITIES

    • Claw: Melee Weapon Attack, +3 to hit, 1 reach 5 ft, one target. d6+1 piercing damage, and the target must succeed in a DC 10 Constitution saving throw or become poisoned for 1 minute.

    DISPOSITION
    Undine are spirits of water and vanity. They dislike dirt, modesty, poverty, and circumspection. They like performance, ostentation, and cleanliness. They really love drowning people.