I commented on [MENTION=6795602]FrogReaver[/MENTION]'s proposed changes in an earlier post (summary; the balance is probably fine but I suspect the changes would not be much fun), but have had some further thoughts while continuing to read the thread.
To compensate for the lack of scaling cantrips I would reward casters with extra spell slots. I'm leaning toward 1 extra spell slot of each level up to level 6 spells. You would gaub the extra spell slot immediately upon reaching the level where you first gain that spell slot. For example a level 5 Wizard would have spell slots of 5 level 1, 4 level 2, 3 level 3 but his cantrips would not scale.
Casters would scale with more spell slots, more powerful spell slots and more powerful spells. So my take is that removing cantrip scaling doesn't go against any of that.
The design principle underlying cantrip scaling is that 5e aims for a balanced rate of power acquisition across classes. All the classes get a power spike at 5th, 11th, and 17th levels, usually from multiple features at once, i.e. rogues at 5th get uncanny dodge but also +1d6 sneak attack. I would argue that this design principle is positive for inter-class balance, the feel of character progression, and the codification of 5e's heroic fantasy sweet spot.
Cantrip scaling is a junior partner in the power spike of full caster classes--the main spikes are 3rd, 6th, and 9th level spells--but cantrip scaling
is part of that power spike. Consequently, if you want to get rid of cantrips and replace them with additional spell slots, you would do the best job at maintaining balance by granting those spell slots at levels 5, 11, and 17, rather than 1, 3, 5, 7, 9, and 11.
One effect of the proposed change is that level 1-4 casters would become noticeably stronger relative to martials. A less obvious effect is that the power acquisition rate of casters would become relatively smoother than that of martials. A messier effect is that you'd need to decide how the change affects 1/2 and 1/3 casters; either they would keep their normal progression, which makes spell slots from multiclassing wonky, or they would get more slots, which would be undesirable for 1/2 casters since they don't get cantrips. A serious drawback, which others have pointed out, is that clerics with cantrip damage features would be significantly penalized relative to clerics with melee damage features. None of these consequences would break the game, but I think you could trade cantrip scaling for spell slots in ways that circumvent these problems.
I think I'd go with more low level slots only. Maybe 3 extra 1st level slots at 5th, 3 extra 2nd level slots at 11th, and 3 extra 3rds at whatever the next level is that cantrips scale up at.
This suggestion strikes me as better suited to 5e's normal power progression, but it also seems a little bit awkward. 3rd level spells are one of those big power spikes that caster classes get--so, while trading +1dX to cantrip damage for three 1st level spell slots makes sense on its face, trading +1dX cantrip damage for three 3rd level spell slots seems like a bit much. Considering that the +1dX cantrip damage is MUCH less relevant at 17th level than at 5th, this seems like an undesirable change.
With my change cantrips would essentially be ribbons.
Yeah, makes sense. They'd just be fancy crossbow attacks--you might even give them 1dX+ability modifier damage (with smaller dice) to drive home that they're just fancy crossbow attacks.
My suggestion:
Don't change the base spell progression. Add/change specific features on a class by class basis that mitigate the removal of cantrip scaling; it's easier to avoid skewing the big interlocking game systems by changing/adding narrower game elements.
Full casters:
- At 5th level, each full caster gets a feature that allows it to cast 3 extra 1st level spells, from a preset list of damaging/healing spells on its class's spell list. These spells are cast as 2nd level spells when used. Once cast, they recover after a long rest. Spells known casters learn one spell from the list to make sure that they can use the feature. These spells are not recoverable with arcane recovery or natural recovery.
- At 11th level, they can cast 5 extra 1st or 2nd level spells, also from a preset list, upcast to 3rd level.
- At 17th level, they can cast 6 extra 1st or 2nd level spells, upcast to 4th level.
Clerics with potent spellcasting:
They upcast spells one level higher than normal with the above feature instead of getting cantrip damage, i.e. 3rd level when they get this feature, 5th level at 17.
Warlocks:
They retain cantrip scaling (as mentioned in the OP).
Paladins and rangers:
No change.
Eldritch knights:
The war magic feature is replaced. The new feature allows them to use any or all of the attacks they make with their attack action to cast a damaging cantrip instead.
Arcane tricksters:
They get no compensation for losing cantrip damage, but GFB and BB are suboptimal compared to two weapon fighting anyway.