Eldritch Blast is really the best damage dealing cantrip.
I don't think so.
Before augmentations*, EB is mostly on par with the other attack cantrips. It's not even the top-damage cantrip (Poison Spray is, with d12), it has a good base damage of d10 compared to most other cantrips, but in general one dice category difference means only 1 point of average damage difference per die, so even at the fourth tier (levels 17+) the average difference is 4 damage points, not a huge advantage at that tier. In compensation for damage lower than d10, the other cantrips all have an extra effect which may not always be useful, but when it is useful it's actually useful at
all levels.
I am actually VERY pleased by how all cantrips were designed pretty balanced against each other. If you were to choose any one attack cantrip among the whole list, there would always be a compelling tactical reason to choose each one. However, all cantrips do NOT normally compete directly into each other, except when a character is considering taking the Magic Initiate feat or has some class features that allows to pick a cantrip from another class spell list.
In the Warlock's general case, Eldritch Blast compete only against Chill Touch and Poison Spray. Now, for a
player character choosing Eldritch Blast is normally slightly better only because
force damage resistance** is rare, while
poison damage resistance is common, and monsters don't often heal. But for a
nonplayer character I'd say that Chill Touch is the best choice, because
it blocks healing, and that against PCs can hurt a lot more than a few points of damage more. Poison Spray is the top-damage dealing cantrip, but the price to pay is the extremely short range, so it can be a good choice only for a melee-oriented Warlock.
If the Warlock has Pact of the Tome however, she can also choose cantrips from other classes, but still there is always a good tactical reason for each one of those.
*Of course, with the proper invocations then Eldritch Blast becomes
better than other cantrips, and here there is quite a variety of potency. Repelling Blast is a nice useful effect, but overall not above of other cantrips bonus effects, so quite balanced IMO. Eldritch Spear is frankly only rarely going to make a difference, since EB range is already long enough in all indoor and most outdoor situations. Agonizing Blast might in fact be a little too good if the Cha bonus applies to each EB ray. The cost to pay is of course not getting other invocations instead.
**It's irrelevant for this discussion, but about this I have sometimes wished that the
Shield spell hadn't been designed specifically against
Magic Missile but more generally against force damage effects.
So good, that our Warlock with Pact of the Tome and Magic Initiate recently mentioned that he never cast any of his other cantrips (except for Guidance).
Bad thought of the day, ignore if you don't want to get mad at me: [sblock]Good for him to like monotony. I know this kind of player: they always think of the game to be monolithically about combat, and combat to be monolithically about damage, they "build" their PC monolithically around that, they have fun for a couple of nights "pwing" the game, then boredom "monolithically" hits them in their back and they blame the game that "sucks" and "needs fixing", while they should only fix themselves. Let him play what he wants and enjoy his later frustration.[/sblock]
Eldritch Blast is removed. In compensation, Warlocks get to add their various Blast invocations to any one-target cantrip that does damage (with one minor change, which is that Spear is tripled range, instead of a flat 300').
Well this is not a bad house rule. There is no real balance problem with it, and it's good when a character option meant to be used in one case can also be used in another. I just don't think you need to ban EB in order to have this house rule.
Additionally, at 2nd level they get the Minor Arcanum: two cantrips from any arcane spell list (Sorcerer, Warlock, Wizard).
I don't really see the reason for this freebie. I'd stick with the 3 default possible Warlock attack cantrips just like other classes stick with their own, and leave the bonus cantrips only to Pact of the Tome.
Yea, I think there's a key question there that has to be answered before you can really houserule EB in any direction: Is the multi-attack component of Eldritch Blast a feature or a bug?
I don't see how you could
not interpret it as a feature. It's definitely written-out explicitly so it must be as intended.
What perhaps is debatable is whether it is intended that the Warlock can target+attack and see the result of each EB beam before choosing the target for the next, or if she's supposed to select all targets when casting but before resolving the attacks (which prevents to always spread the damage optimally).