Help with ideas for warlock pact of the archmage.

[notXanathar]

I want to run a campaign with only arcane casters, and I thought "wouldn't it be could if you had warlock who got there powers from a powerful wizard or sorcerer, like mordenkainen,". However, I have very few ideas, and so have decided to ask for help. All I know for certain is that they should be able to use mystic arcanum as spell slots, able to use them for lower level spells and so on. Beyond that, I know nothing, although I thought it might be cool if this was to the pact of the time what the hexblade is to the pact of the blade.
Absolutely any ideas on the subject would be welcome.

[clash]

Warlocks have the awesome advantage of getting their subclass at level 1 which works great for things like hexblade that change your playstyle. A few ideas off the cuff with no regard for balance:
* Give them a spell book and prepared spellcasting similiar to a wizard. This spellbook could double as their pact of the tome if they choose it.
* Give them a school focus with a single minor ability when using spells of that school
* Give them the ability to affect enemy saves in a limited fashion

[Kane0]

Extra cantrips and spells known are an easy start, from the wizard spell list. Patron bonus spells might even be free.

Augment invocations that grant spells, such as not using a spell slot the first time it's used each Long Rest or being able to pick a second spell of the same level to cast.

Use bonus action to raise your spell attack bonus or DC, with a short rest recharge

Healing or slot recovery when you successfully counterspell/dispel

Ritual casting is already covered, can't do that.

[Bloodcloud]

This is hard. Indeed, warlock already has a book, some overlap on spell list, rituals (better than the wizard), at will low level spells...
expanding on tome boon seems a no-brainer. More known spells?

[Crisis21]

So, a couple of things here:

If we're seriously doing a wizard-themed Warlock we need to break down what the core things of a wizard are:

1) Spellbook. This is something any Warlock can effectively pick up with Pact of the Tome. However, that only allows them to record rituals from any spell list. Rituals are great and all, but it's not quite a true wizard spellbook.
1-a) The ability to record and learn spells at any time, not just as they level up. So... what if one of the features is that your Pact of the Archmage warlock can record non-ritual spells in their grimoire? It would of course lock them into getting Pact of the Tome rather than another pact, but that could just be a price of playing the subrace. Maybe part of the pact they made was that they had to search out new spells that the archmage giving them their power didn't already possess.

2) Access to the biggest spell list in the game. Nothing's going to say 'made a pact with a Wizard' like being able to get your spells from the Wizard spell list in addition to the Warlock spell list.
2-a) Many options for spells to cast. In fact... what about for the archetype's expanded spell list (2 each of 1st, 2nd, 3rd, 4th, and 5th level bonus spells known granted at the level you gain those spell slots) they're able to straight up pick their bonus spells from the Wizard spell list? Same for Mystic Arcanum.

3) High Intelligence. Maybe give your warlock a feature that lets them use their Intelligence as their spellcasting modifier instead of Charisma. INT is a common dump stat, and such a feature could encourage high-INT warlocks by not penalizing them for focusing on a non-optimal stat for their class.
Perhaps in this same vein, you could give a feature that allows the warlock to attempt to figure out the mechanics of a spell just by observing it being cast through an Intelligence (Arcana) check with a DC of 10 + the spell's level. If they succeed enough times, they are able to learn the spell and add it to their book solely through observation.