An archivist is a calling only the quickest of thinkers can undertake. Any who set out to be an archivist has a sole duty to spread and preserve knowledge, with or without consent. They must be able to memorize on the fly, prepare for any and every possible future, and most importantly-- make wise decisions. 

An archivist is a doomsday prepper of the olden ages. Versed in preparing for any and every situation, archivists use their floating bookcases to store extra spells for the coming future.

Wizards Archive

2nd-level Archivist feature

Every archivist keeps their stored spells on them at all times

Before you fall asleep at night, you may expend two spell slots to store away a spell into you magical spell archive. Once per day, as an action, you can cast a stored spell from the archive without expending a spell slot

• You cannot store ritual spells into the archive.
• If the spell casted from the archive requires concentration, it no longer does.
• You can only have ({{3*proficiency}}) spells stored in the archive at any given time.
• The archive loses all stored spells if you fall unconscious

  ---

The archive can appear as one of these variations ([roll]1d4[/roll])

1 Two floating bookcases manifest behind you. You can pull out a book and cast a stored spell from it.

2 A shimmering golden crown appears on your head (of no worth). You can throw one of it's rune inlaid gems to cast a stored spell. The gems regrow on the crown and disappear if they are away from the crown for 15 seconds. The crown is unable to be forcedly removed from your head.

3 You have an infinitely large scroll that you can write down the spells onto. When you read the spell out loud, the writing disappears.

4 You keep a jar of marbles that you can fill with magical spells. If you break the marble under your foot the spell is cast.

 

 

Imperfect recollection

6th-level Archivist feature

You can notice details about enemy attacks. As a reaction, you can memorize any target’s attack within a 120 ft radius. Any time afterwards you become targeted by the attack memorized, you can use a reaction to either provide a +3 to your saving throw or a -3 to the opponent’s attack.

You can use this ability an amount equal to your proficiency modifier per long rest.

You lose memorized attacks after a long rest.

Combat Archivist

10th-level Archivist feature

Once whenever you finish a long rest, you can capture one spell that is targeting you, which is stored among the spells in your archive. You can cast the spell like any other stored spell.
Doing this burns out 2 spell spaces in your archive.

Burnt out spaces in the archive are repaired during a short rest.

Perfect memorization

14th-level Archivist feature

Spending such a large amount of time studying has improved your memory to the point that you can mimic spells used on the battlefield. As an action, you can choose a target that casted a spell last turn and you can cast the same spell as them. 
Mimicking a spell costs 1/3 of the spell level (rounded up) as burnt out storage.
I.e. Sunburst would burn out 3 storage slots, Shatter would burn out 1, Summon Aberration would burn out 2.