[MENTION=6677017]Sword of Spirit[/MENTION] - That certainly looks like a detailed graph. I will take a look a bit later. But I agree that the Warlock
could play more like a spellcaster with the right build. But if the Warlock is supposed to be a true spellcaster in the vein of the sorcerer or warlock, you shouldn't need to build it to make it feel or function like that. Which once again, at least to me, supports the idea that a Warlock shouldn't be a spellcaster. The mechanics don't really support a strong enough identity for players to know how to build a functional warlock build without a good deal of system mastery. To me, that seems like a problem, since a player using a sorcerer or a wizard does not need advanced system mastery to play those classes as they are intended to play by the designers. So, the choice seems to either lean into the spellcasting aspect, or disregard it. I am more in favor of the later.
[MENTION=6798775]Ath-kethin[/MENTION] - That is awesome! I am a huge proponent for reskinning class abilities to make them seem more thematic. Not terribly familiar with Thule, so it might be worth checking out to prune for ideas and inspiration.
1) Eldritch Blast. Maintain it as a cantrip, or move it to be a class feature? Does the warlock still need cantrips? If it becomes a class feature, does it need more invocation support? (I can see a place for a lot of the 3.5 era shape and essence invocations.)
I believe Eldritch Blast should transition from a cantrip to a class feature granted at level 1. I think it should automatically start with gaining Charisma mod to add to damage. And I think that Extra Beams should be a class feature akin to the Fighter. This way, one could theoretically stop it at level 11 with 3 blasts, similar to the fighter, or allow all 4. This would limit abuse of Eldritch Blast since it would no longer advance with character level, but warlock class level. However, I personally would give a 4th attack at level 17, but this is primarily because I also have re-designed the fighter to get a 4th attack at level 17 and created a new capstone based on their chosen Fighting Style. YMMV.
I have also considered the invocations that can shape the Eldritch Blast. I like the idea, but the execution needs to be done carefully or it can quickly become broken.
I also think cantrips need to remain. They add some versatility to the Warlock, and continue the trend of access to at-will spells. I am unsure if cantrip options available should be based on the Patron choice with themed cantrip options, or just based around the Wizard / Sorcerer cantrip list. This helps keep Pact of the Tome useful
2) Patrons. Without Pact Magic, the patron spell list extensions become dead weight. Is there a class feature or some invocations that can leverage them?
Yes! I see this as an opportunity for Patron specific invocations. They were unpopular in the Unearthed Arcana prior to XGtE, so the Patron pre-requisite was scrapped. But I think this is because the Patrons do not have a solid enough identity to support general invocations that would be limited to them. It is too easy to imagine invocations as belonging to any of the patrons. However, this changes if we increase the number of Spell-Per-Rest invocations, since spells are much easier to thematically link to a Patron. So we transition some of the Bonus Spells into invocations that allow it to be used per short rest or long rest (depending on the spell). This gives the Patrons more thematic identity, provides Warlocks with continued support if they do want to cast spells, but doesn't force it upon them since the Patron choice only gives access to these invocations, rather than automatically adding them.
3) Hex. Drop it, make it an invocation, or another class feature?
Yes. I have written Hex as a class ability as follows:
Hex
At 2nd level, you learn to curse your opponents, making your attacks deadlier and hampering their abilities. As a bonus action, you can Hex one opponent with 90 feet. You deal an extra 1d6 necrotic damage to the target whenever you hit it with an attack. Also, choose one ability when use Hex. The target has disadvantage on ability checks made with the chosen ability. This ability lasts for 1 minute.
If the target drops to 0 hit points before the Hex ends, you can use a bonus action on a subsequent turn of yours to curse a new creature.
Once you have used your Hex, you must finish a short or long rest to use it again.
4) Invocations. How many more invocations are needed to make up the loss of the potency of Pact Magic? What other kinds of invocations need to be added? More potent at-will spells (more 1st level, maybe even some 2nd and 3rd level spells) seems like a good design angle, and some more invocations that grant spells 1/LR.
So I have a large number of homebrew invocations that would help expand upon them, including more Pact Specific ones. I think some Warlock exclusive spells would also need to be revamped into invocations (Hellish Rebuke, Armor of Agathys, Arms of Hadar, Hunger of Hadar). As for the number, I'm thinking 15 invocations by level 20. You normally get 8, then add another 4 for your Pact Magic spell slots, then another 3 for Mystic Arcanum. Technically you should have 4 from Mystic Arcanum, but since most invocations are at-will abilities, it probably evens out.
And in order to replicate Mystic Arcanum, we could have some level gated 1/long rest spells available, as well as some of the more powerful at-will abilities. This better supports those that want the Warlock to function like a caster, but also allows the warlock to just be an at-will abilities guy, or somewhere in-between. Additionally, since invocations give per-rest spells rather than spell slots, there is no ability to abuse them with multiclassing Sorcerer or Paladin.