Balancing warlock for a "low"-magic setting

[Sindeloke]

If you were going to limit spellcasting in your world to 5th level spells and below (allowing clerics and wizards etc to upcast into higher level slots, but not learn higher level spells without extensive questing and effort), what would you do with warlocks? Just turn mystic arcanum into actual empty spell slots instead of spells, allowing 1/day/slot upcasting of normal lock spells? Allow the existing warlock slots to scale to 6 or 7 for simplicity? Replace arcanum entirely with a different, thematic feature?

[Vorpalchicken]

Hard level cap of 2!

Everyone will be making pacts with unsavory powers. It will be all the rage! Bard? Pact with the Devil! Paladin? Pact with the Devil! Sorcerer? Pact with the Devil! Cleric? Pact with an ancient inscrutable tentacle-themed demigod!

Nah. I'd actually let them have their Arcanum. It's such a limited list, it won't break anything. It will be cool and it will give people incentives to play Warlocks past level 2!

Or make warlocks NPC only.

[imanidiot]

Make them do a quest comparable to the ones required for other classes at the behest of their parton to get access to thpse spells.

[Prince_Vorrel]

Why not do the exact same thing? Hell! you have a better reason for them to quest for their mystic Arcanum slots. patrons. now instead of having to find a lost ancient powerful spell you literally have to do something of serious importance to your patron...that only gets more and more important as they get higher levels of it.

something for the fiend patron like...

level 6th slot: go kill this powerful warlock who betrayed me. (something that needs to be done and is a lesson to the warlock about betrayal!)
7th: Get a serious cult dedicated to me going. I'm talking real serious cult with at least 250+ members or smaller but with more influential members (nobles, bishops, etc...)
8th: Go to the abyss/hell and retrieve this artifact in the possession of a pit fiend or minor demon lord (really minor or some demon on the cusp of becoming a lord)
9th: Something CRAZY. like go be my general for the blood war and win this major battle for me or go kill Orcus or some **** (With said patrons help/resouces cause at this point the warlock is his god damn right hand at this power level).

Remember patrons are smart bastards...always ask the warlock to do things that he likes to do anyway that still help the patron. "oh hey go kill this really evil dude cause adventurers love to do stuff like that, and give me this powerful weapon he's been hiding"

That way the group just shrugs and agrees to help cause hey...killing evil liches and crap is what they do...who cares if you accidentally end the blood war :P

[Sindeloke]

I think if you can accidentally end the Blood War you're past needing a patron 😆

Questing does seem like a fair solution but it would tend to spotlight the warlock a bit, I'd think. The patron's chief enemy would need to have a really sexy sword for the fighter to pick off his corpse.

[Armored Walrus]

Don't you end up with exactly the same issue if you have the wizard or cleric quest for higher level spells?

[D.U.P.A.]

Just limit them with some spells that do not have such big impact, as some spells (mostly damage ones), are not that world warping.