Etawyrda's Enhanced Familiar (LSoT)

When you cast this spell, you strengthen the bond between a familiar and its summoner, and may imbue a willing familiar with one of the features listed below.

The chosen effect lasts for the duration, unless dispelled, and will continue even if the familiar is reduced to 0 hit points, or is residing in its pocket dimension. If you recast this spell within the duration, the previous effect fades. A familiar can only benefit from one casting of this spell at a time, and the effects do not stack, unless by upcasting as stated below.

If this spell is being cast on a familiar that was not summoned by you, it must be willing or the spell will not take effect, and each effect is tied to the summoner of the familiar, not the caster of this spell.

Heart of the Master: The familiar's hit points, and hit point maximum are increased to your hit point maximum for the duration, and its AC increases by half your character level (rounded down). On a short rest, in addition to any hit dice used on yourself, you may also use up to half of your total hit dice to heal your familiar. When you use this feature, you may heal your familiar an amount equal to the highest roll of the hit dice + your spellcasting modifier.

Elemental Fury: As a bonus action, you can command your familiar to use its action to make a melee spell attack (range 5ft, targeting one creature). It uses your spell attack bonus, and on a hit, deals damage of an amount of d6s equal to your proficiency modifier. The type of damage dealt can be any type except psychic, radiant, and necrotic, you may choose the damage type as part of your bonus action, and the damage type can change each time an attack is made. The manner in which this attack is made (breath, bite, claw, etc) is up to you. When you reach 10th level, you may choose psychic damage, and at 15th level, you may choose radiant or necrotic.

Apprentice of the Arcane: Upon casting this spell, you may select a number of spells to expend, and store them within your familiar. You must choose the specific spells being stored, not just the slot, and the spells may not exceed a total level of your proficiency modifier + 2, or a spell level equal to your proficiency modifier. For the duration, you may use a bonus action to command your familiar to use its action to cast a spell you have stored within it. When casting, it uses your spell attack bonus and spell save DC, but otherwise casts it itself, using the spell slot you expended when storing the spell, and is not able to add your modifier to any damage or healing. If the spell requires concentration, the familiar will cast the spell, but you will be required to maintain concentration. Cantrips may be stored in your familiar, however for the sake of total spell levels being stored, a cantrip will count for 1 level.
At any time within the duration, you may spend 5 minutes with your familiar to expend spells slots and store additional spells within them, however the spells must still not exceed the limit as stated when the spell was cast.

 At Higher Levels. When you cast this spell at 4th level, you may choose up to two effects for the duration. When cast at 5th level, you may take all three effects.